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

lib/main.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
 import 'downloads/downloads_screen.dart';
+import 'library/category_service.dart';
 import 'library/database.dart';
 import 'library/favorites_screen.dart';
 import 'library/library_service.dart';
@@ -13,6 +14,7 @@ import 'player/audio_handler.dart';
 import 'player/mini_player.dart';
 import 'services/logger_service.dart';
 import 'services/offline_mode.dart';
+import 'settings/app_settings.dart';
 import 'shared/theme.dart';
 
 late final MeloAudioHandler _handler;
@@ -20,6 +22,8 @@ late final MeloDb _db;
 late final LibraryService _library;
 late final PlaylistService _playlists;
 late final OfflineMode _offlineMode;
+late final CategoryService _categories;
+late final AppSettings _settings;
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
@@ -29,6 +33,9 @@ Future<void> main() async {
   _playlists = PlaylistService(_db);
   _offlineMode = OfflineMode();
   await _offlineMode.init();
+  _categories = CategoryService(_db);
+  _settings = AppSettings();
+  await _settings.init();
   _handler = await AudioService.init(
     builder: () => MeloAudioHandler(db: _db),
     config: const AudioServiceConfig(
@@ -53,6 +60,8 @@ class MeloApp extends StatelessWidget {
         ChangeNotifierProvider<LibraryService>.value(value: _library),
         ChangeNotifierProvider<PlaylistService>.value(value: _playlists),
         ChangeNotifierProvider<OfflineMode>.value(value: _offlineMode),
+        ChangeNotifierProvider<CategoryService>.value(value: _categories),
+        ChangeNotifierProvider<AppSettings>.value(value: _settings),
       ],
       child: MaterialApp(
         title: 'Melo',