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;
+}

lib/core/providers/database_provider.g.dart

@@ -0,0 +1,51 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'database_provider.dart';
+
+// **************************************************************************
+// RiverpodGenerator
+// **************************************************************************
+
+// GENERATED CODE - DO NOT MODIFY BY HAND
+// ignore_for_file: type=lint, type=warning
+
+@ProviderFor(appDatabase)
+final appDatabaseProvider = AppDatabaseProvider._();
+
+final class AppDatabaseProvider
+    extends $FunctionalProvider<AppDatabase, AppDatabase, AppDatabase>
+    with $Provider<AppDatabase> {
+  AppDatabaseProvider._()
+    : super(
+        from: null,
+        argument: null,
+        retry: null,
+        name: r'appDatabaseProvider',
+        isAutoDispose: false,
+        dependencies: null,
+        $allTransitiveDependencies: null,
+      );
+
+  @override
+  String debugGetCreateSourceHash() => _$appDatabaseHash();
+
+  @$internal
+  @override
+  $ProviderElement<AppDatabase> $createElement($ProviderPointer pointer) =>
+      $ProviderElement(pointer);
+
+  @override
+  AppDatabase create(Ref ref) {
+    return appDatabase(ref);
+  }
+
+  /// {@macro riverpod.override_with_value}
+  Override overrideWithValue(AppDatabase value) {
+    return $ProviderOverride(
+      origin: this,
+      providerOverride: $SyncValueProvider<AppDatabase>(value),
+    );
+  }
+}
+
+String _$appDatabaseHash() => r'59cce38d45eeaba199eddd097d8e149d66f9f3e1';