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.js

@@ -0,0 +1,111 @@
+import 'react-native-gesture-handler';
+import React, { useEffect, useState } from 'react';
+import { View, StatusBar } from 'react-native';
+import { NavigationContainer } from '@react-navigation/native';
+import { createBottomTabNavigator } from '@react-navigation/bottom-tabs';
+import { createStackNavigator } from '@react-navigation/stack';
+import { SafeAreaProvider } from 'react-native-safe-area-context';
+import { GestureHandlerRootView } from 'react-native-gesture-handler';
+import { Ionicons } from '@expo/vector-icons';
+import TrackPlayer, { Event, State } from 'react-native-track-player';
+import { PlaybackService } from './src/audio/service';
+import { setupPlayer } from './src/audio/player';
+import { useStore } from './src/store';
+
+import HomeScreen         from './src/screens/Home';
+import { ArtistsScreen, AlbumsScreen } from './src/screens/Library';
+import ArtistDetailScreen from './src/screens/ArtistDetail';
+import AlbumDetailScreen  from './src/screens/AlbumDetail';
+import SearchScreen       from './src/screens/Search';
+import LoginScreen        from './src/screens/Login';
+import NowPlayingScreen   from './src/screens/NowPlaying';
+import PlayerBar          from './src/components/PlayerBar';
+
+TrackPlayer.registerPlaybackService(() => PlaybackService);
+
+const Tab   = createBottomTabNavigator();
+const Stack = createStackNavigator();
+
+function HomeTabs() {
+  return (
+    <Tab.Navigator
+      screenOptions={({ route }) => ({
+        headerShown: false,
+        tabBarStyle: { backgroundColor: '#1e1e1e', borderTopColor: '#2a2a2a' },
+        tabBarActiveTintColor: '#1db954',
+        tabBarInactiveTintColor: '#888',
+        tabBarIcon: ({ color, size }) => {
+          const icons = { Home: 'home', Artists: 'person', Albums: 'disc', Search: 'search', Settings: 'settings-outline' };
+          return <Ionicons name={icons[route.name] || 'ellipse'} size={size} color={color} />;
+        },
+      })}
+    >
+      <Tab.Screen name="Home"     component={HomeScreen}    options={{ title: 'Start' }} />
+      <Tab.Screen name="Artists"  component={ArtistsScreen} options={{ title: 'Künstler' }} />
+      <Tab.Screen name="Albums"   component={AlbumsScreen}  options={{ title: 'Alben' }} />
+      <Tab.Screen name="Search"   component={SearchScreen}  options={{ title: 'Suche' }} />
+      <Tab.Screen name="Settings" component={LoginScreen}   options={{ title: 'Einstellungen' }} />
+    </Tab.Navigator>
+  );
+}
+
+function RootNavigator() {
+  const currentTrack = useStore(s => s.currentTrack);
+  return (
+    <View style={{ flex: 1 }}>
+      <Stack.Navigator screenOptions={{ headerShown: false }}>
+        <Stack.Screen name="Tabs"         component={HomeTabs} />
+        <Stack.Screen name="NowPlaying"   component={NowPlayingScreen} options={{ presentation: 'modal' }} />
+        <Stack.Screen name="AlbumDetail"  component={AlbumDetailScreen} />
+        <Stack.Screen name="ArtistDetail" component={ArtistDetailScreen} />
+      </Stack.Navigator>
+      {currentTrack && <PlayerBar />}
+    </View>
+  );
+}
+
+export default function App() {
+  const { loadConfig } = useStore();
+  const [ready, setReady] = useState(false);
+
+  useEffect(() => {
+    (async () => {
+      await loadConfig();
+      await setupPlayer();
+      setReady(true);
+    })();
+  }, []);
+
+  useEffect(() => {
+    const sub1 = TrackPlayer.addEventListener(Event.PlaybackActiveTrackChanged, e => {
+      if (e.track) useStore.getState().setCurrentTrack(e.track._song || e.track);
+    });
+    const sub2 = TrackPlayer.addEventListener(Event.PlaybackState, e => {
+      useStore.getState().setIsPlaying(e.state === State.Playing);
+    });
+    return () => { sub1.remove(); sub2.remove(); };
+  }, []);
+
+  if (!ready) return null;
+
+  return (
+    <GestureHandlerRootView style={{ flex: 1 }}>
+      <SafeAreaProvider>
+        <StatusBar barStyle="light-content" backgroundColor="#121212" />
+        <NavigationContainer theme={{
+          dark: true,
+          colors: {
+            primary:      '#1db954',
+            background:   '#121212',
+            card:         '#1e1e1e',
+            text:         '#ffffff',
+            border:       '#2a2a2a',
+            notification: '#1db954',
+          },
+        }}>
+          <RootNavigator />
+        </NavigationContainer>
+      </SafeAreaProvider>
+    </GestureHandlerRootView>
+  );
+}