Untidy 0.6 beta

This prerelease adds the first simple, real offline download implementation on top of the validated 0.5 watch UX.

Highlights

Bumps Android app metadata to versionName=0.6.0-beta.1, versionCode=6.
Keeps the validated Now Playing pull-over context menu and working crown volume behavior.
Adds real current-track download from Now Playing using sanctioned TIDAL usage=DOWNLOAD manifests and local Media3 cache fill.
Adds Track Context download flow: Download / Downloading… / Downloaded.
Adds album and playlist download actions as simple batches over track-level offline units.
Downloads screen continues as the discovery/play/remove surface for local tracks.
Includes the shared offline unit model for track/album/playlist UX consistency.

Ric physically validated the one-track offline scenario before this release request.
Release gate passed:
- lintDebug
- testDebugUnitTest
- assembleRelease
- git diff --check
Release string scan confirmed the new app version string and no old debug-only offline-unavailable copy surfaced.

Album/playlist offline download flows are implemented but still need Ric's physical wrist validation.
Download progress is intentionally simple for this beta.