feat(01-01): add core infrastructure, localization, and Wave 0 tests

- AppDatabase with schemaVersion 1, in-memory executor for testing
- Database provider with @Riverpod(keepAlive: true)
- AppTheme with sage green seed ColorScheme and warm surface overrides
- ThemeNotifier with SharedPreferences persistence, defaults to system
- Full German ARB localization (15 keys) with proper umlauts
- Minimal main.dart with ProviderScope placeholder
- Drift schema v1 captured via make-migrations
- All .g.dart files generated via build_runner
- Wave 0 tests: database (3), color scheme (6), theme (3), localization (2) -- 14 total, all passing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/l10n/app_localizations_de.dart

@@ -0,0 +1,70 @@
+// ignore: unused_import
+import 'package:intl/intl.dart' as intl;
+import 'app_localizations.dart';
+
+// ignore_for_file: type=lint
+
+/// The translations for German (`de`).
+class AppLocalizationsDe extends AppLocalizations {
+  AppLocalizationsDe([String locale = 'de']) : super(locale);
+
+  @override
+  String get appTitle => 'HouseHoldKeaper';
+
+  @override
+  String get tabHome => 'Übersicht';
+
+  @override
+  String get tabRooms => 'Räume';
+
+  @override
+  String get tabSettings => 'Einstellungen';
+
+  @override
+  String get homeEmptyTitle => 'Noch nichts zu tun!';
+
+  @override
+  String get homeEmptyMessage =>
+      'Lege zuerst einen Raum an, um Aufgaben zu planen.';
+
+  @override
+  String get homeEmptyAction => 'Raum erstellen';
+
+  @override
+  String get roomsEmptyTitle => 'Hier ist noch alles leer!';
+
+  @override
+  String get roomsEmptyMessage => 'Erstelle deinen ersten Raum, um loszulegen.';
+
+  @override
+  String get roomsEmptyAction => 'Raum erstellen';
+
+  @override
+  String get settingsSectionAppearance => 'Darstellung';
+
+  @override
+  String get settingsThemeLabel => 'Farbschema';
+
+  @override
+  String get themeSystem => 'System';
+
+  @override
+  String get themeLight => 'Hell';
+
+  @override
+  String get themeDark => 'Dunkel';
+
+  @override
+  String get settingsSectionAbout => 'Über';
+
+  @override
+  String get aboutAppName => 'HouseHoldKeaper';
+
+  @override
+  String get aboutTagline => 'Dein Haushalt, entspannt organisiert.';
+
+  @override
+  String aboutVersion(String version) {
+    return 'Version $version';
+  }
+}