Linthra 0.1.0-alpha.1 — first manually testable alpha.

Working in this build:
- Pick a local folder, scan it (Storage Access Framework on Android), and
  list the discovered tracks; the choice and catalog persist across restarts.
- Local audio playback with an up-next queue.
- Background playback with a media notification and lock-screen / Bluetooth /
  headset controls.
- Android Auto: a browsable Library and Queue from the car screen.
- Jellyfin: connect to your own server, sign in, sync your library, and
  stream tracks. The session token is stored encrypted; passwords are never
  saved.
- Explicit, user-initiated offline downloads of Jellyfin tracks, with a
  "Wi-Fi only" option. Nothing downloads automatically.

Alpha, so expect rough edges. Known limits: no tag/metadata parsing yet
(tracks show file names), track-level downloads only, single Jellyfin server,
optimistic connectivity detection, and no saved playlists. Not on F-Droid yet.
