feat(library): add Artist and Album browsing tabs

- Library screen now has a TabBar (Alle / Künstler / Alben) inside a
  DefaultTabController; scan/permission/error banners stay above the
  TabBarView so they remain visible regardless of the active sub-tab
- New lib/library/song_grouping.dart: pure, unit-tested functions
  groupByArtist/groupByAlbum (null artist/album -> "Unbekannt" /
  "Unbekanntes Album", alphabetically sorted) plus albumArtistLabel
  ("Verschiedene Interpreten" when an album mixes artists)
- New lib/library/artist_list.dart + album_list.dart: StreamBuilder on
  MeloDb.watchSongs(), grouped client-side, tapping a row pushes a
  SongList screen for that artist/album; album rows show a CoverImage
  thumbnail from the first song's coverPath
- Existing "Alle" tab content (recently-added strip, full song list,
  empty state) preserved unchanged, moved into _AllSongsTab

Genre browsing is intentionally out of scope: the Songs table has no
genre column and neither scanner extracts genre metadata — needs a
schema migration + scanner work in a future task.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

lib/library/album_list.dart

@@ -0,0 +1,69 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../shared/cover.dart';
+import 'database.dart';
+import 'song_grouping.dart';
+import 'song_list.dart';
+
+/// Bibliotheks-Tab: Album-Übersicht, gruppiert aus [MeloDb.watchSongs].
+class AlbumListScreen extends StatelessWidget {
+  const AlbumListScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final db = context.read<MeloDb>();
+    return StreamBuilder<List<Song>>(
+      stream: db.watchSongs(),
+      builder: (context, snapshot) {
+        final songs = snapshot.data ?? const [];
+        if (songs.isEmpty) {
+          return const Center(
+            child: Text('Keine Alben', style: TextStyle(color: Colors.white54)),
+          );
+        }
+        final grouped = groupByAlbum(songs);
+        final albums = grouped.keys.toList();
+        return ListView.builder(
+          itemCount: albums.length,
+          itemBuilder: (context, i) {
+            final album = albums[i];
+            final albumSongs = grouped[album]!;
+            final coverPath = albumSongs.first.coverPath;
+            return ListTile(
+              leading: CoverImage(
+                artUri: coverPath != null ? Uri.file(coverPath) : null,
+                size: 48,
+                radius: 6,
+              ),
+              title: Text(album, maxLines: 1, overflow: TextOverflow.ellipsis),
+              subtitle: Text('${albumArtistLabel(albumSongs)} · ${albumSongs.length} Songs',
+                  maxLines: 1, overflow: TextOverflow.ellipsis),
+              onTap: () => Navigator.push(
+                context,
+                MaterialPageRoute(
+                  builder: (_) => _AlbumSongsScreen(album: album, songs: albumSongs),
+                ),
+              ),
+            );
+          },
+        );
+      },
+    );
+  }
+}
+
+/// Zeigt alle Songs eines Albums.
+class _AlbumSongsScreen extends StatelessWidget {
+  const _AlbumSongsScreen({required this.album, required this.songs});
+  final String album;
+  final List<Song> songs;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: Text(album)),
+      body: SongList(songs),
+    );
+  }
+}