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/player.tsx

@@ -6,6 +6,7 @@ import { Pressable, Text, View, type LayoutChangeEvent } from 'react-native';
 import { player } from '../player/controller';
 import { usePlayerState } from '../player/hooks';
 import { colors } from '../theme';
+import { DownloadButton } from '../ui/downloads';
 import { Cover, Icon, Screen, formatDuration } from '../ui/kit';
 
 /** Полноэкранный плеер (вид cover): обложка, прогресс с перемоткой, лайк / дизлайк, управление. */
@@ -37,7 +38,7 @@ export default function PlayerScreen() {
             <Icon name="keyboard-arrow-down" size={32} color={colors.text} />
           </Pressable>
           <Text style={{ color: colors.text2, fontSize: 13 }}>{st.wave ? t('wave.title') : t('mobile.now_playing')}</Text>
-          <View style={{ width: 32 }} />
+          <DownloadButton track={track} />
         </View>
 
         <View style={{ alignItems: 'center' }}>