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/app.json

@@ -0,0 +1,38 @@
+{
+  "expo": {
+    "name": "NovaSonic",
+    "slug": "NovaSonic_Mobile",
+    "version": "1.0.0",
+    "orientation": "portrait",
+    "icon": "./assets/icon.png",
+    "userInterfaceStyle": "dark",
+    "ios": {
+      "supportsTablet": true,
+      "infoPlist": {
+        "UIBackgroundModes": ["audio"]
+      }
+    },
+    "android": {
+      "adaptiveIcon": {
+        "backgroundColor": "#121212",
+        "foregroundImage": "./assets/adaptive-icon.png"
+      },
+      "package": "com.honeyy.novasonic",
+      "permissions": [
+        "android.permission.FOREGROUND_SERVICE",
+        "android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK"
+      ]
+    },
+    "plugins": [
+      [
+        "react-native-track-player",
+        {
+          "androidPackage": "com.guichaguri.trackplayer"
+        }
+      ]
+    ],
+    "web": {
+      "favicon": "./assets/favicon.png"
+    }
+  }
+}