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>

test/core/database/database_test.dart

@@ -0,0 +1,31 @@
+import 'package:drift/native.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:household_keeper/core/database/database.dart';
+
+void main() {
+  group('AppDatabase', () {
+    late AppDatabase db;
+
+    setUp(() {
+      db = AppDatabase(NativeDatabase.memory());
+    });
+
+    tearDown(() async {
+      await db.close();
+    });
+
+    test('opens successfully with in-memory executor', () {
+      expect(db, isNotNull);
+    });
+
+    test('has schemaVersion 1', () {
+      expect(db.schemaVersion, equals(1));
+    });
+
+    test('can be closed without error', () async {
+      await db.close();
+      // If we reach here, close succeeded. Re-create for tearDown.
+      db = AppDatabase(NativeDatabase.memory());
+    });
+  });
+}