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Thymeleaf or Angular4 with Springboot

I've mostly worked with Springboot framework only with JSP to cover the things I need for the website part. Now, I've got a project to do that mostly revolves, if not all, around the website and I have a couple of questions.
Just to be clear first, I don't have experience either with Thymeleaf or Angular so whichever I pick will be the first time (I think thymeleaf syntax would be easier for me to handle).
Ok so the main goal in my mind is not to render the whole page every time I load data from the backend, so I figure I would have like a header/content/footer parts where every time I click something only the content part would change. Also, I would like the possibility for a loader to show and go away when the content part is changing. This web application will need to be secure for users that register.
I've searched the web to investigate both frameworks, but can not seem to find the right answer so I can continue with my development
I do not mind creating separate REST services in later development if some other platform needs to hook up to the service if I decide to go with Thymeleaf.What do you guys think in what direction should I go, Thymeleaf or Angular? Any help and/or discussion would be much appreciated.
I am sorry if this seems like a general question, but I just need some basic guidelines to start with. Cheers!

I think transitioning to ThymeLeaf is probably going to be the easiest for you, but Angular is a great choice as well, and from there it is up to you. Would you rather use mostly expressions similar to JSTL expressions, but in Spring's SpEL language, then use ThymeLeaf, or would you rather use JavaScript, then use Angular. It just a user's preference for what you are doing.
The fragmenting portion (header, footer, body, etc..) is native to both frameworks. It just depends on which one you want to use. Whatever you go with, to load specific sections while not rendering the others, is going to require AJAX and for you to feel comfortable with how the template frameworks work.
I would suggest reading up on both of them to figure out which one you feel more comfortable with.
Angular
Thymeleaf
Both of them have great documentation for beginners and the Baeldung and Mkyong have good walkthrus for ThymeLeaf. Angular's documentation I found good enough on its own.
For a loader, you can do with simple CSS and JS. There is a ton of demos out there on how to do full screen loaders and how to turn them on or off with JS or CSS. IHateTomatoes has a good article about how to build a full screen loader, that has a No JS fallback option and should give you a good starting point.
Your point about it needing to be secure is a whole other monster. I would look into Spring Security. It's relatively straightforward, especially when using Spring Boot. If you want it can control the users session and assist in preventing session jacking, add CSRF to prevent cross site forgery, control permissions to urls and on down the line, or not. It all just depends.
Either way, don't randomly stab at security, it will end up in something that you feel is secure but it is not, which leaves you and your users in a very bad spot. Again, Baeldung has a great walkthru on the user registration and login process that can help get you started with Spring security and how to tie everything together.
Pretty high level answer, but hopefully gave you some good starting points and some resources.

Build apps decoupling frontend from the backend.
Always build apps following the "The API-first approach"
The API-first approach involves setting up the foundation of your app, which is the application programming interface
For me the differences between Thymeleaf and Angular:
Using Thymeleaf: You don't need to create Restful/web service endpoints on your frontend side because you just need to make calls to the backend from the frontend itself.
Using Angular: Besides of your Restful/ web service endpoints on your backend side, you have to build Restful/ web services endpoints on your frontend side because you don't want to expose direct access from Javascript code to the backend.
Hope this helps and happy coding time!

Client-side or server-side framework?

My project would be a kind of craiglist, a site where users could post anouncements (evereday-life objects, cars, flat etc.). So, authentication, profile page, content creation, display the for-sale objects etc.
I have developed a very large part of the backend: I have a RESTful API in three-tier architecture developed in java. It makes the link with the db, to provide me with different urls and send me the relevant JSON.
URLs example:
http://api.mywebsite.fr/user?userid=1 sends me back:
{"user": {"username": "jdoe1234", "email", "jdoe1234#gmail.com"}}
I have urls for all actions performed on the entire site (anouncement creation, last data updates ... everything, and I've carefully declared them POST, GET, UPDATE, DELETE, etc.). There is also oAuth to protect the API from queries that are not allowed for the token.
That's all for the "server" aspect, I think that there is no problem with that.
But if all the actions are managed by the webservice, I do not see the interest that could bring me a big server-side framework like Symfony/cakePHP, Zend, etc., to make HTTP requests on my different entry points, retrieve JSON and populate the HTML.
So I looked at client framework, like Angular, Ember and so on. At first, it seemed very suitable for my case: possibility of http requests, manage what to do in case of success or error, directly exploit the resulting JSON to populate the view etc.
I didn't even manage to make my choice between angularjs and Ember, both being very similar, but with the release of Angular v2, I fear the maintainability of v1 (if I choose Angular, it will be v1 , because the majority of tutorials and questions relate to Angular 1.X).
I don't know if I'm doing the right thing by choosing client-side framework, I am afraid that they 'brident' (not sure of that word, sorry) me. Plus, it's fully instantiated in the browser, so the user can change absolutely all code and data I provide. That seems weird to me.
I want to be absolutely sure of the technology that I use in case I make this application available to the public for example. I want to do things properly, in order to avoid maintainability or security problems.
Summary: With the things I already have (webservice / api), is it a good idea to use a client framework like Angular or should I stay on big server-side framework like Symfony/Zend etc? Knowing that I position myself in the context in which this platform would be massively used (craiglist comparable traffic).

I'd say - depends whether you want to be more frontend guy or backend guy in future. If you want to be full stack developer then it doesn't apply.
In my opinion, both Symfony/Zend or other big server-side frameworks aren't so exciting as dynamic frontend JavaScript frameworks like Ember/Angular/React.
Also, if you have already RESTful API and OAuth authentication implemented in backend part I'd go with Ember. Why? Ember Data is great tool for talking to backend API. It's mature, it lazily loads records when they're needed and it's very customizable.
it's fully instantiated in the browser,so the user can change
absolutely all code and data I provide...
Ember has built in security like sanitizing data which is rendered in it's templating language - HTMLBars. Also, there's CORS and content security policy (CSP) standard which is implemented in Ember.
I want to be absolutely sure of the technology that I use in case I
make this application available to the public for example. I want to
do things properly, in order to avoid maintainability or security
problems .
In Ember you can create mature, secure, production-ready applications, but you need to comfortable with your Ember skills to some degree to build such ambitious web application, but it's part of building every application.
With the things that i already have(webservice / api), is it a good
idea to use a client framework like Angular?
Yes, it's very popular solution to use MEAN stack or go with Ember + RESTful API.
Why should I choose Ember instead of Angular (which have a larger
community/tutorials/answered questions) ?
Angular has larger community/tutorials/answered questions, but when I started some side project with Angular to learn its possible advantages over Ember, I was surprised how there was no consensus in it's community for doing one thing. So, instead of fast search how to declare and use directives (I think it was the thing that confused me) I have to do another research which way is the best. Also, there are lots of ways to setup project (where to put custom directives, different Angular objects) and you have to do another research which one to choose. I ended up using repo healthy-gulp-angular as my template, but you can see it hasn't been updated for 8 months, but I think during these 8 months Angular had a lot of changes and I'm not sure if this repo is the best choice.
In Ember you have Ember CLI tool which is built with Convention over Configuration principle. You have also Ember Data which utilizes JSON API standard - if you don't have JSON API compliant server side right now, you can write custom adapter to normalize server responses or change how backend replies. In Ember you don't have all that headache and different best solutions to do 1 basic thing depending who you ask.
What means "Single page application" ?
Single-page application is basically a page which doesn't have to reload all assets and HTML when you navigate. It's advantage over PHP - when user moves to another location he downloads only new data for that route. More info here.
Does those frameworks allow me to create real routes ? (
www.myapp/profil/userid etc )
Yes, of course. You don't even need # in your URL. With simple rewrite rule and small amount of logic for profile route and specified path profile/:userid, when user will open URL www.myapp/profile/userid he will be automatically taken to profile route, and userid would be interpreted as route parameter, so you can take this userid and find user record from the store in model hook.

Client = speed, Server = stability
JS frameworks updates once per week
Non-Js back-end once per year
Client side depends to behavior depending on browser
Back is related only on machine but not on environment
I chose FE coz I tired to debug code by writing variables values to database to actually see what is going on in controllers -_-

Why do we need javascript mvc?

When would one go for using Javascript MVC ? I mean why there was a
need for JS-MVC ?
Is it only because this design pattern was famous in other languages, for code maintenance,readability and many web apps are shipping client side ?
How does it helps a developer, tester and end-user easing their
tasks ?
Any use-case where JS-MVC is suited best and any case where it is
not at all required ?

Question 1,2,4
My view is that you should apply any kind of design pattern only and only if you really need it. Simply because design patterns aren't easy after all. They will add complexity to your solution, but they'll also provide you the benefit of (normally) being established and working solutions for specific kind of problems.
Whether to use or not use them really depends on what you are going to construct and how complex it is. That said, I'm probably not going to structure my 200 lines jQuery plugin using the MVC pattern, maybe...
But at work we create single page applications where we have 2-5 devs working on a project the same time, projects of the amount of 500 days. In such environments things can get complex very quickly and if you don't follow any kind of proper structuring you're lost.
I hope that should answer your question 1 and 2.
For question 3:
The end-user hopefully gets a better quality app with less bugs, but usually he shouldn't even notice (nor does he care) about the underlying architecture of an app.
MVC helps the developer
as an orientation when navigating in the app's source code. Consider an app of 3000 lines of code in a single source file and then when having 4 devs working on it at the same time. A mess, right? When having an MVC app which normally also use routings etc...you normally already know by looking at the route in the url where the corresponding controller lies in the source code and where you should put your hands on
for easier testing. Separation of concerns is always beneficial when applying unit testing practices because you can much easier test your controller because it isn't directly couple to data or presentational stuff (like HTML code etc). And just as a side note, you should definitely unit-test your JavaScript code, definitely!!
maintenance: which is the result of the previous points somehow
I'm not sure which kind of figure you intend by "tester". If he writes automated tests at the unit or acceptance level, then the benefits from before pretty much hold as well. If he's testing the app as a whole in the sense of navigating around, taking the app as a black box by testing its reaction to different kind of inputs, then MVC doesn't really play a big role for him. It's like for the end-user.
So I hope I was able to clarify some things for you. But as said, never just follow a pattern because people follow it but only 'cause it gives you any kind of benefits.

MVC is used to structure your code especially if you are working a lot with client side rendering. Without structure your code gets complex very fast.
User experience for end user is much better when you don't have to wait for the whole page refresh if you only need partial update.
It helps to navigate through the code much faster if you have well defined structure of your code.
JS MVC frameworks are best used where you have complex systems and you want to provide better user experience by using AJAX and partial page reloads or updates. Usually you would not use MVC for Personal website, blogs or stuff like that...

Do I have to use a Backend when using Backbone.js?

I want to develop a relatively simple application that calculates some value based on several inputs. I dont want a backend, all the calculation can be done in the browser.
Im a little new to JavaScript and WebApps and I came across Backbone.js.
I really like the MVC design, however, they mention a backend a lot. My question:
Is a backend server absolutely required?
Is a backend server optional but without one there isn't much point in backbone.
Or will backbone will really help me out?

Backend is not required.
Backbone can fully work without any backend if your application doesn't require one.
That depends on your application. If you want to retrieve value of some inputs and calculate a result then Backbone won't do that for you - it will help you structure your code. If you app is simple and don't need support for models, views and collections or routing, then there is no point in using Backbone. Hard to answer this question.
For example: Classic todo example application doesn't use any backend.

Backbone.js implements fetch(), save(), destroy() etc. methods on models automatically performing appropriate AJAX requests and parsing response. So it has a strong support for backend via REST services, but it is optional.
You can still use models, views, routers and events without any server-side code. Just don't call REST methods (or override them at your wish).

You can use localStorage for persistence (you'd have to implement this yourself or find it on the web, like here) but if you don't even need that then you don't need to use any of the persistence methods in backbone.
Backbone is meant to help you structure a medium-large sized application (js-wise), so it doesn't become unmaintainable jQuery spaghetti. With short applications (js-wise) it's really an overkill unless you are trying to learn how backbone works.
Note with js-wise I mean the client side code, if you had a huge backend but the only js would be something that focuses some form, it would not even count as a short application (js-wise).

You can use backbone.js without a backend. However you obviously won't be able to store or retrieve data. Backbone may still be useful for keeping your code organized, however it really shines when you want to separate presentation logic from logic that manipulates your data, which is a goal of the MVC pattern. Generally your data will be stored on and retrieved from a backend.
If you want to play around with data persistence, try out backlift.com. [disclosure, I work on backlift.com] We've tried to make it easy to get a backbone app up-and-running without having to setup a server or deal with compiling templates.

Best way to separate data from DOM

I have this time tracking / task managment tool I'm working on called Task Ranger. Basically you make a tree of tasks and sub-tasks, click on whichever task you're currently working on, and your time for that task is tracked.
I want to add a feature where the user can look at their times for a specific date range (ie. "What did I spend my time on last week?") I'm thinking I want to have a tab that I can click to jump from the "Main" view to the "History" view.
The problem is that right now I'm persisting my data by simply storing everything in the DOM and dumping the entire html structure into localStorage. In order to get the history view to work I'm going to need to separate the data from the html, right? I guess an MVC framework is the typical way of doing that, right? I was looking at Backbone, but I'm feeling like it might be a little overkill for this. I'm thinking maybe I should roll my own simple little thing. What do you think?
I'm using Javascript + Jquery for all of this right now.

Backbone is about as lightweight as you can get MVC. If I were to write my own micro MVC framework it would convert to backbone within a week.
I would highly recommend Backbone over writing your own because it's well structured and loosely coupled. You can only use the features of backbone that you want. Since your already including jQuery there is little overhead of including backbone aswell.
Backbone is great for structuring and organizing your code.
You can use the backbone-localstorage adapter to save your models to localStorage.
You can then save a backlog of all your models or get them from a RESTful server for your history view.
A solid alternative to backbone would be spine which is also a lightweight MVC library. Spine has a more traditional MVC attitude and is only 2kB