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/app_theme.dart

@@ -0,0 +1,45 @@
+import 'package:flutter/material.dart';
+
+class AppTheme {
+  AppTheme._();
+
+  static ThemeData lightTheme() {
+    final colorScheme = ColorScheme.fromSeed(
+      seedColor: const Color(0xFF7A9A6D),
+      brightness: Brightness.light,
+      dynamicSchemeVariant: DynamicSchemeVariant.tonalSpot,
+    ).copyWith(
+      surface: const Color(0xFFF5F0E8),
+      surfaceContainerLowest: const Color(0xFFFAF7F2),
+      surfaceContainerLow: const Color(0xFFF2EDE4),
+      surfaceContainer: const Color(0xFFEDE7DC),
+      surfaceContainerHigh: const Color(0xFFE7E0D5),
+      surfaceContainerHighest: const Color(0xFFE0D9CE),
+    );
+
+    return ThemeData(
+      useMaterial3: true,
+      colorScheme: colorScheme,
+    );
+  }
+
+  static ThemeData darkTheme() {
+    final colorScheme = ColorScheme.fromSeed(
+      seedColor: const Color(0xFF7A9A6D),
+      brightness: Brightness.dark,
+      dynamicSchemeVariant: DynamicSchemeVariant.tonalSpot,
+    ).copyWith(
+      surface: const Color(0xFF2A2520),
+      surfaceContainerLowest: const Color(0xFF1E1A16),
+      surfaceContainerLow: const Color(0xFF322D27),
+      surfaceContainer: const Color(0xFF3A342E),
+      surfaceContainerHigh: const Color(0xFF433D36),
+      surfaceContainerHighest: const Color(0xFF4D463F),
+    );
+
+    return ThemeData(
+      useMaterial3: true,
+      colorScheme: colorScheme,
+    );
+  }
+}

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';
+    }
+  }
+}

lib/core/theme/theme_provider.g.dart

@@ -0,0 +1,62 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'theme_provider.dart';
+
+// **************************************************************************
+// RiverpodGenerator
+// **************************************************************************
+
+// GENERATED CODE - DO NOT MODIFY BY HAND
+// ignore_for_file: type=lint, type=warning
+
+@ProviderFor(ThemeNotifier)
+final themeProvider = ThemeNotifierProvider._();
+
+final class ThemeNotifierProvider
+    extends $NotifierProvider<ThemeNotifier, ThemeMode> {
+  ThemeNotifierProvider._()
+    : super(
+        from: null,
+        argument: null,
+        retry: null,
+        name: r'themeProvider',
+        isAutoDispose: true,
+        dependencies: null,
+        $allTransitiveDependencies: null,
+      );
+
+  @override
+  String debugGetCreateSourceHash() => _$themeNotifierHash();
+
+  @$internal
+  @override
+  ThemeNotifier create() => ThemeNotifier();
+
+  /// {@macro riverpod.override_with_value}
+  Override overrideWithValue(ThemeMode value) {
+    return $ProviderOverride(
+      origin: this,
+      providerOverride: $SyncValueProvider<ThemeMode>(value),
+    );
+  }
+}
+
+String _$themeNotifierHash() => r'060f7d104905995da8c785d2f50f15a4db2d7022';
+
+abstract class _$ThemeNotifier extends $Notifier<ThemeMode> {
+  ThemeMode build();
+  @$mustCallSuper
+  @override
+  void runBuild() {
+    final ref = this.ref as $Ref<ThemeMode, ThemeMode>;
+    final element =
+        ref.element
+            as $ClassProviderElement<
+              AnyNotifier<ThemeMode, ThemeMode>,
+              ThemeMode,
+              Object?,
+              Object?
+            >;
+    element.handleCreate(ref, build);
+  }
+}