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/theme/theme_provider.dart

@@ -0,0 +1,53 @@
+import 'package:flutter/material.dart';
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+part 'theme_provider.g.dart';
+
+const _themeModeKey = 'theme_mode';
+
+@riverpod
+class ThemeNotifier extends _$ThemeNotifier {
+  @override
+  ThemeMode build() {
+    _loadPersistedThemeMode();
+    return ThemeMode.system;
+  }
+
+  Future<void> _loadPersistedThemeMode() async {
+    final prefs = await SharedPreferences.getInstance();
+    final persisted = prefs.getString(_themeModeKey);
+    if (persisted != null) {
+      state = _themeModeFromString(persisted);
+    }
+  }
+
+  Future<void> setThemeMode(ThemeMode mode) async {
+    state = mode;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_themeModeKey, _themeModeToString(mode));
+  }
+
+  static ThemeMode _themeModeFromString(String? value) {
+    switch (value) {
+      case 'light':
+        return ThemeMode.light;
+      case 'dark':
+        return ThemeMode.dark;
+      case 'system':
+      default:
+        return ThemeMode.system;
+    }
+  }
+
+  static String _themeModeToString(ThemeMode mode) {
+    switch (mode) {
+      case ThemeMode.light:
+        return 'light';
+      case ThemeMode.dark:
+        return 'dark';
+      case ThemeMode.system:
+        return 'system';
+    }
+  }
+}