feat(mobile): offline downloads, persistent event queue and offline start (M5)

- Downloads queue (2 parallel, Wi-Fi only by default) stores files under
  the app's document directory and track metadata in SQLite; Collection
  gets a Downloads tab, album and player screens get download buttons.
- Player plays downloaded files and local covers; My Wave falls back to
  a shuffle of downloaded tracks when the server is unreachable.
- Playback events that fail on the network are kept in SQLite and sent
  when the connection returns.
- AuthSession can open offline with the cached user and fetches an
  access token before the first request once online.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/mobile/package.json

@@ -26,6 +26,7 @@
     "expo-linking": "~7.1.7",
     "expo-router": "~5.1.11",
     "expo-secure-store": "~14.2.4",
+    "expo-sqlite": "~15.2.14",
     "expo-status-bar": "~2.2.3",
     "i18next": "^25.2.1",
     "js-sha256": "^1.0.0",