feat: add cover art to folders

[martinshoob]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• Folders now display the cover art of the first track they contain.
• Updated the folder list UI to display thumbnails.
• Implemented cover picking logic in a similar way to existing album track covers.

Tests performed

• Tested on Samsung S24 Ultra in all the available themes. Tested different system text sizes.

Before & after preview

Before

After

Closes the following issue(s)

• Closes Add album covers on folders #100

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.