Linthra 0.1.0-alpha.15 — QA & stabilization pass after real-device testing.

Fixed:
- Settings/About version now matches the published release.
- Cancelling or clearing a download mid-fetch no longer lets it reappear as
  "downloaded" (and no stray file is left behind).
- Pre-cache no longer double-fetches the same track if asked twice at once.
- "Add to playlist" now reports the number of songs actually added (duplicates
  already in the playlist are no longer counted).
- Signing out of Jellyfin now clears that account's synced favorites and stale
  sync status, so they can't linger or cross over to another account.
- Jellyfin diagnostics no longer report a previous server after a failed test of
  a different address.

Also: broader automated test coverage and a manual Android QA checklist.

Still an alpha — expect rough edges. Not on F-Droid yet.
