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,
+    );
+  }
+}