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/l10n/localization_test.dart

@@ -0,0 +1,56 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:household_keeper/l10n/app_localizations.dart';
+
+void main() {
+  group('AppLocalizations (German)', () {
+    late AppLocalizations l10n;
+
+    testWidgets('loads German localization and displays tabHome',
+        (tester) async {
+      late AppLocalizations capturedL10n;
+
+      await tester.pumpWidget(
+        MaterialApp(
+          localizationsDelegates: AppLocalizations.localizationsDelegates,
+          supportedLocales: const [Locale('de')],
+          locale: const Locale('de'),
+          home: Builder(
+            builder: (context) {
+              capturedL10n = AppLocalizations.of(context);
+              return Text(capturedL10n.tabHome);
+            },
+          ),
+        ),
+      );
+
+      await tester.pumpAndSettle();
+
+      // Verify the rendered text contains the expected German string
+      expect(find.text('\u00dcbersicht'), findsOneWidget);
+    });
+
+    testWidgets('all critical keys are non-empty', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          localizationsDelegates: AppLocalizations.localizationsDelegates,
+          supportedLocales: const [Locale('de')],
+          locale: const Locale('de'),
+          home: Builder(
+            builder: (context) {
+              l10n = AppLocalizations.of(context);
+              return const SizedBox.shrink();
+            },
+          ),
+        ),
+      );
+
+      await tester.pumpAndSettle();
+
+      expect(l10n.appTitle, isNotEmpty);
+      expect(l10n.tabHome, isNotEmpty);
+      expect(l10n.tabRooms, isNotEmpty);
+      expect(l10n.tabSettings, isNotEmpty);
+    });
+  });
+}