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.g.dart

@@ -0,0 +1,19 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'database.dart';
+
+// ignore_for_file: type=lint
+abstract class _$AppDatabase extends GeneratedDatabase {
+  _$AppDatabase(QueryExecutor e) : super(e);
+  $AppDatabaseManager get managers => $AppDatabaseManager(this);
+  @override
+  Iterable<TableInfo<Table, Object?>> get allTables =>
+      allSchemaEntities.whereType<TableInfo<Table, Object?>>();
+  @override
+  List<DatabaseSchemaEntity> get allSchemaEntities => [];
+}
+
+class $AppDatabaseManager {
+  final _$AppDatabase _db;
+  $AppDatabaseManager(this._db);
+}