[Feature Request]: configure text size separately from thumbnail size

### 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 has only a full page zoom, which assumes that there is an optimal relation between the fonts and thumbnail size that works for all or most users. As a result, thumbnails in a grid of playlists are huge and annoying compared to their captions, and zooming does nothing to change that. https://github.com/FreeTubeApp/FreeTube/issues/4605

### Proposed Solution

Separate the text and thumbnail size controls, at least in the settings.
Since everything is controlled by CSS, it should be easy to specify a base font size in pixels and a thumbnail size in pixels or ms or ns.
Themes could be made external CSS files, so the user could specify the sizes for specific contexts like the playlist windows.

### Alternatives Considered

Add buttons to +/- font size and thumbnails on the top panel.

### Issue Labels

ease of use improvement, visual improvement

### Additional Information

_No response_