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/core/providers/database_provider.dart

@@ -0,0 +1,11 @@
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+import 'package:household_keeper/core/database/database.dart';
+
+part 'database_provider.g.dart';
+
+@Riverpod(keepAlive: true)
+AppDatabase appDatabase(Ref ref) {
+  final db = AppDatabase();
+  ref.onDispose(db.close);
+  return db;
+}