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

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+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());
+    });
+  });
+}

test/core/theme/color_scheme_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:household_keeper/core/theme/app_theme.dart';
+
+void main() {
+  group('AppTheme ColorScheme', () {
+    test('lightTheme has Brightness.light', () {
+      final theme = AppTheme.lightTheme();
+      expect(theme.colorScheme.brightness, equals(Brightness.light));
+    });
+
+    test('darkTheme has Brightness.dark', () {
+      final theme = AppTheme.darkTheme();
+      expect(theme.colorScheme.brightness, equals(Brightness.dark));
+    });
+
+    test('both themes use sage green seed (primary in green hue range)', () {
+      final lightPrimary = HSLColor.fromColor(
+        AppTheme.lightTheme().colorScheme.primary,
+      );
+      final darkPrimary = HSLColor.fromColor(
+        AppTheme.darkTheme().colorScheme.primary,
+      );
+
+      // Sage green hue is approximately 100-150 degrees
+      expect(lightPrimary.hue, inInclusiveRange(80, 160));
+      expect(darkPrimary.hue, inInclusiveRange(80, 160));
+    });
+
+    test('light surface color is warm stone (0xFFF5F0E8)', () {
+      final theme = AppTheme.lightTheme();
+      expect(theme.colorScheme.surface, equals(const Color(0xFFF5F0E8)));
+    });
+
+    test('dark surface color is warm charcoal (0xFF2A2520), not cold gray', () {
+      final theme = AppTheme.darkTheme();
+      expect(theme.colorScheme.surface, equals(const Color(0xFF2A2520)));
+    });
+
+    test('both themes use Material 3', () {
+      expect(AppTheme.lightTheme().useMaterial3, isTrue);
+      expect(AppTheme.darkTheme().useMaterial3, isTrue);
+    });
+  });
+}

test/core/theme/theme_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:household_keeper/core/theme/theme_provider.dart';
+
+void main() {
+  group('ThemeNotifier', () {
+    setUp(() {
+      SharedPreferences.setMockInitialValues({});
+    });
+
+    test('defaults to ThemeMode.system', () {
+      final container = ProviderContainer();
+      addTearDown(container.dispose);
+
+      final themeMode = container.read(themeProvider);
+      expect(themeMode, equals(ThemeMode.system));
+    });
+
+    test('setThemeMode(dark) updates state to dark', () async {
+      final container = ProviderContainer();
+      addTearDown(container.dispose);
+
+      await container.read(themeProvider.notifier).setThemeMode(
+        ThemeMode.dark,
+      );
+      expect(container.read(themeProvider), equals(ThemeMode.dark));
+    });
+
+    test('setThemeMode(light) updates state to light', () async {
+      final container = ProviderContainer();
+      addTearDown(container.dispose);
+
+      await container.read(themeProvider.notifier).setThemeMode(
+        ThemeMode.light,
+      );
+      expect(container.read(themeProvider), equals(ThemeMode.light));
+    });
+  });
+}