Kategorien (mehrere je Song), Metadaten-Ansicht mit Expertenmodus

- DB-Schema 4: Tabelle song_categories + Spalte categories_edited
- Kategorien kommen aus dem Genre-Tag (Trennung an ; , | /), manuell
  bearbeitbar; von Hand gesetzte Kategorien ueberlebt der naechste Scan
- Songzeile zeigt "Kuenstler | Kategorie1 - Kategorie2"
- Einstellung "Gleiche Kategorie = gleiches Coverbild" (Standard an)
- Metadaten-Sheet mit ausklappbarem Expertenmodus
- Neu: categories.dart, category_service.dart, song_detail_sheet.dart,
  app_settings.dart

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011snPXPafPC5i87o68H14W7

test/library/my_music_screen_test.dart

@@ -2,12 +2,14 @@ import 'package:drift/drift.dart';
 import 'package:drift/native.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:melo/library/category_service.dart';
 import 'package:melo/library/database.dart';
 import 'package:melo/library/favorites_screen.dart';
 import 'package:melo/library/library_service.dart';
 import 'package:melo/library/my_music_screen.dart';
 import 'package:melo/library/playlist_service.dart';
 import 'package:melo/player/audio_handler.dart';
+import 'package:melo/settings/app_settings.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
@@ -19,6 +21,8 @@ Widget _wrap(MeloDb db, LibraryService lib, PlaylistService playlists,
       ChangeNotifierProvider<LibraryService>.value(value: lib),
       ChangeNotifierProvider<PlaylistService>.value(value: playlists),
       Provider<MeloAudioHandler>.value(value: handler),
+      ChangeNotifierProvider<CategoryService>(create: (_) => CategoryService(db)),
+      ChangeNotifierProvider<AppSettings>(create: (_) => AppSettings()),
     ],
     child: MaterialApp(home: Scaffold(body: child)),
   );