[Feature Request]: Add identifier for when app is in 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

FreeTube gets a lot of issue reports by users who do not know that they have fullscreen mode on and/or do not know how to exit it. This is referring to the whole app — not a video — being open in fullscreen.

### Proposed Solution

Add some kind of identifier to the user experience that fullscreen is on and can be disabled. E.g., a toggle in Theme Settings to enable/disable fullscreen.

### Alternatives Considered

I'm not familiar with how other apps have handled this problem, but there could be other good solutions to this. If anyone does have a better idea in mind that another app employs, please share it.

### Issue Labels

ease of use improvement, improvement to existing feature

### Additional Information

_No response_