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/artist_list.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import 'database.dart';
+import 'song_grouping.dart';
+import 'song_list.dart';
+
+/// Bibliotheks-Tab: Künstler-Übersicht, gruppiert aus [MeloDb.watchSongs].
+class ArtistListScreen extends StatelessWidget {
+  const ArtistListScreen({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 Künstler', style: TextStyle(color: Colors.white54)),
+          );
+        }
+        final grouped = groupByArtist(songs);
+        final artists = grouped.keys.toList();
+        return ListView.builder(
+          itemCount: artists.length,
+          itemBuilder: (context, i) {
+            final artist = artists[i];
+            final artistSongs = grouped[artist]!;
+            return ListTile(
+              title: Text(artist, maxLines: 1, overflow: TextOverflow.ellipsis),
+              subtitle: Text('${artistSongs.length} Songs'),
+              onTap: () => Navigator.push(
+                context,
+                MaterialPageRoute(
+                  builder: (_) => _ArtistSongsScreen(artist: artist, songs: artistSongs),
+                ),
+              ),
+            );
+          },
+        );
+      },
+    );
+  }
+}
+
+/// Zeigt alle Songs eines Künstlers.
+class _ArtistSongsScreen extends StatelessWidget {
+  const _ArtistSongsScreen({required this.artist, required this.songs});
+  final String artist;
+  final List<Song> songs;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: Text(artist)),
+      body: SongList(songs),
+    );
+  }
+}