[Feature Request]: Header Bar Button To Toggle Fullscreen

### Guidelines

- [x] I have searched the issue tracker for [open](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aopen+is%3Aissue) and [closed](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aissue+is%3Aclosed) issues that are similar to the feature request I want to file, without success.
- [x] I have searched the [documentation](https://docs.freetubeapp.io/) for information that matches the description of the feature request I want to file, without success.
- [x] This issue contains only one feature request.

### Problem Description

Hello!

Due to the fact that FreeTube sometimes launches in fullscreen, I would like a button to toggle it on the header bar.

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### Proposed Solution

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I would like if there was a button to toggle fullscreen on the main header bar.
This is similar to the 'Create New Window' header bar button.

This would allow those on tablet setups such as the Steam Deck to exit the application without having to switch virtual desktops.
It also prevents those users from getting stuck in the application and being unable to exit without restarting their device.

Thankyou!

### Alternatives Considered

F11 button works, but that is much less convenient on some setups that are without a keyboard.

### Issue Labels

ease of use improvement

### Additional Information

I am using Arch Linux on a Steam Deck, and setting a window rule to never fullscreen also applies this to fullscreened videos themselves. This shows the top bar on KDE whilst the video is playing. This is very annoying.

No files or tests are named. Start by locating the existing “Create New Window” header bar button and the current fullscreen handling, then determine how the header bar can expose a matching toggle. Done means users on setups such as the Steam Deck can enter and exit application fullscreen without a keyboard while fullscreen video behavior remains unaffected.

Written by the indexing model from the issue text.