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>

test/library/song_grouping_test.dart

@@ -0,0 +1,112 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:melo/library/database.dart';
+import 'package:melo/library/song_grouping.dart';
+
+Song _song({
+  required String id,
+  required String title,
+  String? artist,
+  String? album,
+}) {
+  return Song(
+    id: id,
+    path: '/$id.mp3',
+    title: title,
+    artist: artist,
+    album: album,
+    dateAddedMs: 0,
+    updatedAtMs: 0,
+    deleted: false,
+  );
+}
+
+void main() {
+  group('groupByArtist', () {
+    test('gruppiert mehrere Künstler getrennt', () {
+      final songs = [
+        _song(id: '1', title: 'A', artist: 'Bob'),
+        _song(id: '2', title: 'B', artist: 'Alice'),
+        _song(id: '3', title: 'C', artist: 'Alice'),
+      ];
+
+      final grouped = groupByArtist(songs);
+
+      expect(grouped.keys, ['Alice', 'Bob']);
+      expect(grouped['Alice']!.length, 2);
+      expect(grouped['Bob']!.length, 1);
+    });
+
+    test('gruppiert Songs ohne Künstler unter "Unbekannt"', () {
+      final songs = [
+        _song(id: '1', title: 'A', artist: null),
+        _song(id: '2', title: 'B', artist: null),
+      ];
+
+      final grouped = groupByArtist(songs);
+
+      expect(grouped.keys, [unbekannterKuenstler]);
+      expect(grouped[unbekannterKuenstler]!.length, 2);
+    });
+
+    test('sortiert Künstlernamen alphabetisch', () {
+      final songs = [
+        _song(id: '1', title: 'A', artist: 'Zebra'),
+        _song(id: '2', title: 'B', artist: 'Anton'),
+        _song(id: '3', title: 'C', artist: 'Mitte'),
+      ];
+
+      final grouped = groupByArtist(songs);
+
+      expect(grouped.keys.toList(), ['Anton', 'Mitte', 'Zebra']);
+    });
+
+    test('leere Liste ergibt leere Map', () {
+      expect(groupByArtist(const []), isEmpty);
+    });
+  });
+
+  group('groupByAlbum', () {
+    test('gruppiert mehrere Alben getrennt', () {
+      final songs = [
+        _song(id: '1', title: 'A', album: 'Best Of'),
+        _song(id: '2', title: 'B', album: 'Anthology'),
+      ];
+
+      final grouped = groupByAlbum(songs);
+
+      expect(grouped.keys.toList(), ['Anthology', 'Best Of']);
+    });
+
+    test('gruppiert Songs ohne Album unter "Unbekanntes Album"', () {
+      final songs = [_song(id: '1', title: 'A', album: null)];
+
+      final grouped = groupByAlbum(songs);
+
+      expect(grouped.keys, [unbekanntesAlbum]);
+    });
+
+    test('leere Liste ergibt leere Map', () {
+      expect(groupByAlbum(const []), isEmpty);
+    });
+  });
+
+  group('albumArtistLabel', () {
+    test('zeigt gemeinsamen Künstler, wenn alle Songs von ihm stammen', () {
+      final songs = [
+        _song(id: '1', title: 'A', artist: 'Alice', album: 'X'),
+        _song(id: '2', title: 'B', artist: 'Alice', album: 'X'),
+      ];
+
+      expect(albumArtistLabel(songs), 'Alice');
+    });
+
+    test('zeigt "Verschiedene Interpreten" bei unterschiedlichen Künstlern', () {
+      final songs = [
+        _song(id: '1', title: 'A', artist: 'Alice', album: 'X'),
+        _song(id: '2', title: 'B', artist: 'Bob', album: 'X'),
+      ];
+
+      expect(albumArtistLabel(songs), 'Verschiedene Interpreten');
+    });
+  });
+}