Add offline playlist caching with live download status (+ justfile)

[Aron-Magg]

Summary

Adds the ability to keep selected Navidrome/Subsonic playlists available
offline on Android, with live per-song download status and on-disk size
reporting. Also adds a justfile to streamline common dev/build commands.

Offline playlist caching

• Keep offline toggle in the playlist options menu downloads every song
  of the playlist and persists the choice in settings (cachedPlaylists).
• Kept playlists are re-synced automatically on launch when a connection
  is available: newly added songs are downloaded, removed ones are pruned.
  When offline, the cached copy is left untouched.
• Live download status per song: a percentage while downloading, then a
  cloud icon once cached. Reactive via a small useSyncExternalStore store,
  so the UI updates without reopening the screen.
• Cached playlists show the disk space they occupy next to the
  minutes/songs stats, and a cloud indicator in the Playlists tab.
• Remove from offline unmarks a playlist and frees its songs, preserving
  any songs shared with another kept playlist.

Implementation notes

• New app/utils/offlineSync.native.js (web no-op) holds add/remove/sync +
  garbage-collection logic.
• app/components/OfflineSync.js is a headless component mounted in the
  provider tree that runs the sync on launch.
• downloadSong now reports progress via expo-file-system
createDownloadResumable and emits start/progress/finish to the store.
• Reuses the existing on-disk cache and player URL resolution
  (convertToTrack already prefers the local file when cached).
• New i18n keys: Keep offline, Remove from offline (en + it; other
  locales fall back to en).

Platform support

Offline playlist caching relies on the device file system and is Android
only; the web build keeps using its service-worker caching (no-op shims).

Tooling

• Adds a justfile wrapping the npm scripts and local Gradle builds
  (just build-android runs expo prebuild + gradlew assembleRelease,
  no EAS account required). EAS builds are kept under build-eas-*.

Testing

• just lint (project ESLint) passes.
• Local release APK builds successfully via just build-android.
• Manual runtime testing of the full flow is still recommended.