feat(library): Navidrome-Server-Tab in Bibliothek

- Neuer 4. Tab: '🌐 Server' zeigt Alben vom Musikserver
- Album-Liste laden und anzeigen (AsyncBuilder)
- Tap auf Album → alle Songs in Queue laden + abspielen
- Graceful Fallback wenn Server nicht verbunden
- Tests: 70/70 grün
- Analyzer: ✅ keine Fehler

Phase 1 P0-Feature: Album-Browser vom Server

lib/library/library_screen.dart

@@ -1,7 +1,9 @@
+import 'package:audio_service/audio_service.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
 import '../player/audio_handler.dart';
+import '../services/navidrome_service.dart';
 import '../shared/cover.dart';
 import 'album_list.dart';
 import 'artist_list.dart';
@@ -21,7 +23,7 @@ class LibraryScreen extends StatelessWidget {
   Widget build(BuildContext context) {
     final lib = context.watch<LibraryService>();
     return DefaultTabController(
-      length: 3,
+      length: 4,
       child: Scaffold(
         appBar: AppBar(
           title: const Text('Bibliothek'),
@@ -42,6 +44,7 @@ class LibraryScreen extends StatelessWidget {
               Tab(text: 'Alle'),
               Tab(text: 'Künstler'),
               Tab(text: 'Alben'),
+              Tab(text: '🌐 Server'),
             ],
           ),
         ),
@@ -85,12 +88,13 @@ class LibraryScreen extends StatelessWidget {
                   ],
                 ),
               ),
-            const Expanded(
+            Expanded(
               child: TabBarView(
                 children: [
-                  _AllSongsTab(),
+                  const _AllSongsTab(),
-                  ArtistListScreen(),
+                  const ArtistListScreen(),
-                  AlbumListScreen(),
+                  const AlbumListScreen(),
+                  const _NavidromeTab(),
                 ],
               ),
             ),
@@ -238,3 +242,149 @@ class _Empty extends StatelessWidget {
     );
   }
 }
+
+class _NavidromeTab extends StatefulWidget {
+  const _NavidromeTab();
+
+  @override
+  State<_NavidromeTab> createState() => _NavidromeTabState();
+}
+
+class _NavidromeTabState extends State<_NavidromeTab> {
+  late final NavidromeService _nav = NavidromeService();
+  List<SubsonicAlbum> _albums = [];
+  bool _loading = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _nav.ladeGespeicherteZugangsdaten().then((_) {
+      if (_nav.istVerbunden && mounted) {
+        _loadAlbums();
+      }
+    });
+  }
+
+  Future<void> _loadAlbums() async {
+    if (!_nav.istVerbunden) {
+      return;
+    }
+    setState(() => _loading = true);
+    try {
+      final alben = await _nav.getAlben(anzahl: 50);
+      if (mounted) {
+        setState(() {
+          _albums = alben;
+          _loading = false;
+        });
+      }
+    } catch (e) {
+      if (mounted) {
+        setState(() => _loading = false);
+      }
+      debugPrint('Fehler beim Laden der Alben: $e');
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (!_nav.istVerbunden) {
+      return Center(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            const Icon(Icons.cloud_off, size: 64, color: Colors.white24),
+            const SizedBox(height: 12),
+            const Text('Musikserver nicht verbunden',
+                style: TextStyle(color: Colors.white54)),
+            const SizedBox(height: 12),
+            const Text('Gehe zu Einstellungen → Navidrome zum Verbinden',
+                style: TextStyle(color: Colors.white30, fontSize: 12)),
+          ],
+        ),
+      );
+    }
+
+    if (_loading) {
+      return const Center(
+        child: CircularProgressIndicator(color: Colors.redAccent),
+      );
+    }
+
+    if (_albums.isEmpty) {
+      return Center(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            const Icon(Icons.album, size: 64, color: Colors.white24),
+            const SizedBox(height: 12),
+            const Text('Keine Alben geladen',
+                style: TextStyle(color: Colors.white54)),
+            const SizedBox(height: 16),
+            FilledButton.icon(
+              icon: const Icon(Icons.refresh),
+              label: const Text('Erneut versuchen'),
+              onPressed: _loadAlbums,
+            ),
+          ],
+        ),
+      );
+    }
+
+    return ListView.builder(
+      padding: const EdgeInsets.all(12),
+      itemCount: _albums.length,
+      itemBuilder: (context, i) {
+        final album = _albums[i];
+        return Card(
+          margin: const EdgeInsets.only(bottom: 8),
+          child: ListTile(
+            leading: Icon(
+              Icons.album,
+              size: 48,
+              color: Colors.redAccent.withValues(alpha: 0.5),
+            ),
+            title: Text(album.name),
+            subtitle: Text('${album.songCount} Songs'),
+            onTap: () => _playAlbum(album),
+          ),
+        );
+      },
+    );
+  }
+
+  Future<void> _playAlbum(SubsonicAlbum album) async {
+    final handler = context.read<MeloAudioHandler>();
+    try {
+      final songs = await _nav.getSongs(album.id);
+      if (songs.isEmpty) {
+        if (!mounted) {
+          return;
+        }
+        final messenger = ScaffoldMessenger.of(context);
+        messenger.showSnackBar(
+          const SnackBar(content: Text('Album hat keine Songs')),
+        );
+        return;
+      }
+      final items = songs
+          .map((s) => MediaItem(
+            id: _nav.streamUrl(s.id).toString(),
+            title: s.titel,
+            artist: s.kuenstler,
+            album: s.album,
+            duration: Duration(seconds: s.dauerSekunden),
+          ))
+          .toList();
+      await handler.loadPlaylist(items);
+    } catch (e) {
+      if (!mounted) {
+        return;
+      }
+      final messenger = ScaffoldMessenger.of(context);
+      messenger.showSnackBar(
+        SnackBar(content: Text('Fehler: $e')),
+      );
+    }
+  }
+}