[Feature Request]: Add gamepad navigation support

### 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

I am trying to get Freetube set up on my Steam Deck and, while it works in both Desktop and Game mode using the keyboard and mouse, I would argue that it would be convenient while in gaming mode to have the option to control the app using whatever gamepad I have connected to the Deck

### Proposed Solution

Add native Gamepad navigation on the Freetube app for Linux (Flatpack), and check that it works in Steam’s Big Picture mode / Steam Deck’s Gaming Mode

### Alternatives Considered

Remapping keyboard navigation to the Gamepad using Steam Input

### Issue Labels

ease of use improvement, new optional setting

### Additional Information

_No response_

No files, tests, or entry points are named. Start by locating the app's input and navigation handling, then determine the required Linux/Flatpak gamepad behavior. Done means gamepad navigation works in Steam Big Picture mode and Steam Deck Gaming Mode.

Written by the indexing model from the issue text.