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/database/database.dart

@@ -0,0 +1,22 @@
+import 'package:drift/drift.dart';
+import 'package:drift_flutter/drift_flutter.dart';
+import 'package:path_provider/path_provider.dart';
+
+part 'database.g.dart';
+
+@DriftDatabase(tables: [])
+class AppDatabase extends _$AppDatabase {
+  AppDatabase([QueryExecutor? executor]) : super(executor ?? _openConnection());
+
+  @override
+  int get schemaVersion => 1;
+
+  static QueryExecutor _openConnection() {
+    return driftDatabase(
+      name: 'household_keeper',
+      native: const DriftNativeOptions(
+        databaseDirectory: getApplicationSupportDirectory,
+      ),
+    );
+  }
+}