ChairLift: Add Brewfile bundles for Wallpaper Slideshow and Video Wallpaper #1159

Summary

Expose Damask (wallpaper slideshow) and Hidamari (video wallpaper) as installable bundles in ChairLift's UI under the Applications page.

Background & Architecture

• ChairLift reads maintainer defaults from /usr/share/chairlift/config.yml (applications_page.brew_bundles_group points to /usr/share/ublue-os/homebrew).
• ChairLift does NOT allow inventing new pages or groups in config.yml (strict schema validation fails closed on unknown keys).
• Instead, brew_bundles_group dynamically scans immediate *.Brewfile files in /usr/share/ublue-os/homebrew/ and renders each file as a bundle entry with an Install action.
• Uninstalling is handled via ChairLift's flatpak_user_group / flatpak_system_group, which enumerates installed Flatpaks and provides standard uninstall capabilities.

Implementation Steps

1. Add system_files/shared/usr/share/ublue-os/homebrew/wallpaper-slideshow.Brewfile:

   flatpak "app.drey.Damask"

2. Add system_files/shared/usr/share/ublue-os/homebrew/video-wallpaper.Brewfile:

   flatpak "io.github.jeffshee.Hidamari"

3. Ensure permissions and path placement follow repo conventions (shared layer, world-readable 0644).

Testing & Verification

• Run bash scripts/validate-brewfiles.sh.
• Run pytest tests/test_chairlift_config.py.
• Verify bats tests in tests/test_native_recipes.bats continue passing.