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/src/components/PlayerBar.js

@@ -0,0 +1,110 @@
+import React from 'react';
+import { View, Text, Image, TouchableOpacity, StyleSheet } from 'react-native';
+import { useNavigation } from '@react-navigation/native';
+import { usePlaybackState, useProgress, State } from 'react-native-track-player';
+import { Ionicons } from '@expo/vector-icons';
+import { useStore } from '../store';
+import { togglePlayPause, skipNext } from '../audio/player';
+
+export default function PlayerBar() {
+  const navigation    = useNavigation();
+  const { currentTrack, isPlaying } = useStore();
+  const playbackState = usePlaybackState();
+  const { position, duration } = useProgress(500);
+
+  if (!currentTrack) return null;
+
+  const playing = playbackState.state === State.Playing;
+  const progress = duration > 0 ? position / duration : 0;
+
+  return (
+    <TouchableOpacity
+      style={styles.container}
+      activeOpacity={0.9}
+      onPress={() => navigation.navigate('NowPlaying')}
+    >
+      {/* Progress bar at top */}
+      <View style={styles.progressTrack}>
+        <View style={[styles.progressFill, { width: `${progress * 100}%` }]} />
+      </View>
+
+      <View style={styles.row}>
+        {currentTrack.coverArt ? (
+          <Image
+            source={{ uri: currentTrack.artwork }}
+            style={styles.art}
+          />
+        ) : (
+          <View style={[styles.art, styles.artFallback]}>
+            <Ionicons name="musical-note" size={18} color="#888" />
+          </View>
+        )}
+
+        <View style={styles.info}>
+          <Text style={styles.title} numberOfLines={1}>{currentTrack.title}</Text>
+          <Text style={styles.artist} numberOfLines={1}>{currentTrack.artist}</Text>
+        </View>
+
+        <TouchableOpacity style={styles.btn} onPress={togglePlayPause}>
+          <Ionicons
+            name={playing ? 'pause' : 'play'}
+            size={24}
+            color="#fff"
+          />
+        </TouchableOpacity>
+        <TouchableOpacity style={styles.btn} onPress={skipNext}>
+          <Ionicons name="play-skip-forward" size={22} color="#fff" />
+        </TouchableOpacity>
+      </View>
+    </TouchableOpacity>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    backgroundColor: '#1e1e1e',
+    borderTopWidth: 1,
+    borderTopColor: '#2a2a2a',
+  },
+  progressTrack: {
+    height: 2,
+    backgroundColor: '#333',
+  },
+  progressFill: {
+    height: 2,
+    backgroundColor: '#1db954',
+  },
+  row: {
+    flexDirection: 'row',
+    alignItems: 'center',
+    paddingHorizontal: 12,
+    paddingVertical: 8,
+    gap: 10,
+  },
+  art: {
+    width: 44,
+    height: 44,
+    borderRadius: 4,
+  },
+  artFallback: {
+    backgroundColor: '#2a2a2a',
+    alignItems: 'center',
+    justifyContent: 'center',
+  },
+  info: {
+    flex: 1,
+  },
+  title: {
+    color: '#fff',
+    fontSize: 14,
+    fontWeight: '600',
+  },
+  artist: {
+    color: '#aaa',
+    fontSize: 12,
+    marginTop: 2,
+  },
+  btn: {
+    padding: 6,
+  },
+});