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/src/app/album/[id].tsx

@@ -5,6 +5,7 @@ import { useTranslation } from 'react-i18next';
 import { FlatList, Pressable, Text, View } from 'react-native';
 
 import { useSession } from '../../lib/session';
+import { downloads } from '../../offline/downloads';
 import { player } from '../../player/controller';
 import { usePlayerState } from '../../player/hooks';
 import { colors } from '../../theme';
@@ -46,6 +47,7 @@ export default function AlbumScreen() {
                   </Text>
                 </Pressable>
                 <Button title={t('player.play')} icon="play-arrow" onPress={() => void player.playList(a.tracks, 0, ctx)} />
+                <Button title={t('mobile.dl_album')} variant="secondary" icon="download" onPress={() => void downloads.enqueue(a.tracks)} />
               </>
             )}
           </View>