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>

drift_schemas/household_keeper/drift_schema_v1.json

@@ -0,0 +1,10 @@
+{
+  "_meta": {
+    "description": "This file contains a serialized version of schema entities for drift.",
+    "version": "1.3.0"
+  },
+  "options": {
+    "store_date_time_values_as_text": false
+  },
+  "entities": []
+}