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/screens/ArtistDetail.js

@@ -0,0 +1,97 @@
+import React, { useEffect, useState } from 'react';
+import {
+  View, Text, FlatList, Image, TouchableOpacity,
+  StyleSheet, ActivityIndicator, ScrollView,
+} from 'react-native';
+import { useNavigation, useRoute } from '@react-navigation/native';
+import { Ionicons } from '@expo/vector-icons';
+import { api, apiCached, dispArtistById, dispAlbumName, coverUrl, getArtistImage, cacheArtistSortNames } from '../api';
+import AlbumCard from '../components/AlbumCard';
+
+export default function ArtistDetailScreen() {
+  const navigation = useNavigation();
+  const { artist } = useRoute().params;
+  const [albums,  setAlbums]  = useState([]);
+  const [img,     setImg]     = useState(null);
+  const [loading, setLoading] = useState(true);
+
+  useEffect(() => {
+    (async () => {
+      try {
+        const r = await apiCached('getArtist', `&id=${artist.id}`);
+        const ndArtist = r.artist;
+        if (ndArtist) cacheArtistSortNames([ndArtist]);
+        setAlbums(ndArtist?.album || []);
+      } catch {}
+      setLoading(false);
+      getArtistImage(artist.id).then(setImg).catch(() => {});
+    })();
+  }, [artist.id]);
+
+  const name = dispArtistById(artist.id, artist.name);
+
+  return (
+    <View style={styles.container}>
+      {/* Hero */}
+      <View style={styles.hero}>
+        {img ? (
+          <Image source={{ uri: img }} style={styles.heroImg} />
+        ) : (
+          <View style={[styles.heroImg, styles.heroFallback]}>
+            <Text style={{ fontSize: 64 }}>🎤</Text>
+          </View>
+        )}
+        <TouchableOpacity style={styles.backBtn} onPress={() => navigation.goBack()}>
+          <Ionicons name="chevron-back" size={24} color="#fff" />
+        </TouchableOpacity>
+        <View style={styles.heroOverlay}>
+          <Text style={styles.heroName}>{name}</Text>
+          <Text style={styles.heroSub}>{albums.length} Alben</Text>
+        </View>
+      </View>
+
+      {loading ? (
+        <View style={styles.center}><ActivityIndicator color="#1db954" /></View>
+      ) : (
+        <FlatList
+          data={albums}
+          keyExtractor={a => a.id}
+          numColumns={2}
+          renderItem={({ item }) => (
+            <View style={{ flex: 1, padding: 8 }}>
+              <AlbumCard
+                album={item}
+                size={(require('react-native').Dimensions.get('window').width / 2) - 20}
+                onPress={() => navigation.navigate('AlbumDetail', { album: item })}
+              />
+            </View>
+          )}
+          contentContainerStyle={{ padding: 4 }}
+        />
+      )}
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: { flex: 1, backgroundColor: '#121212' },
+  hero: { width: '100%', height: 220, position: 'relative' },
+  heroImg: { width: '100%', height: 220 },
+  heroFallback: {
+    backgroundColor: '#1e1e1e',
+    alignItems: 'center', justifyContent: 'center',
+  },
+  heroOverlay: {
+    position: 'absolute', bottom: 0, left: 0, right: 0,
+    padding: 16, paddingBottom: 20,
+    background: 'transparent',
+  },
+  heroName: { color: '#fff', fontSize: 24, fontWeight: '800', textShadowColor: '#000', textShadowRadius: 8 },
+  heroSub:  { color: '#ddd', fontSize: 13, marginTop: 2 },
+  backBtn: {
+    position: 'absolute', top: 44, left: 12,
+    backgroundColor: 'rgba(0,0,0,0.5)', borderRadius: 20,
+    padding: 6,
+  },
+  center: { flex: 1, alignItems: 'center', justifyContent: 'center', paddingTop: 40 },
+});