feat: add Expo mobile app (mobile/)

Full React Native app using react-native-track-player for native
audio — fixes background freeze, autoplay stopping, stuck play
button, and enables Android Auto in future. Includes:

- Navidrome API layer (ported from src/api.js)
- Zustand global store with SecureStore config persistence
- react-native-track-player with background playback service
- Bottom tab navigation: Home, Artists, Albums, Search, Settings
- Screens: Home, Library (Artists/Albums), ArtistDetail, AlbumDetail,
  Search, NowPlaying, Login/Settings
- Persistent PlayerBar with progress indicator
- Romaji/sort-tag display support carried over from web app

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mobile/package.json

@@ -0,0 +1,32 @@
+{
+  "name": "novasonic_mobile",
+  "version": "1.0.0",
+  "main": "index.js",
+  "dependencies": {
+    "@expo/vector-icons": "^15.1.1",
+    "@react-native-community/slider": "^5.2.0",
+    "@react-navigation/bottom-tabs": "^7.17.2",
+    "@react-navigation/native": "^7.3.0",
+    "@react-navigation/stack": "^7.10.2",
+    "expo": "~56.0.9",
+    "expo-file-system": "^56.0.7",
+    "expo-image": "^56.0.10",
+    "expo-secure-store": "^56.0.4",
+    "expo-status-bar": "~56.0.4",
+    "react": "19.2.3",
+    "react-native": "0.85.3",
+    "react-native-gesture-handler": "^3.0.0",
+    "react-native-reanimated": "^4.4.1",
+    "react-native-safe-area-context": "^5.8.0",
+    "react-native-screens": "^4.25.2",
+    "react-native-track-player": "^4.1.2",
+    "zustand": "^5.0.14"
+  },
+  "scripts": {
+    "start": "expo start",
+    "android": "expo start --android",
+    "ios": "expo start --ios",
+    "web": "expo start --web"
+  },
+  "private": true
+}