[Developer] #215 AI frame-suggest vertical slice (mock LlmService)

설계서대로 구현. flutter_gemma 실제 통합은 OQ-1 (모델 URL+SHA) 확정 후.
v1은 LlmService 추상 + ModelLifecycle (다운로드/SHA/purge) + Riverpod
providers + 다이얼로그 + Settings 화면까지. main.dart 가 MockLlmService 를
override 해 모든 경로가 graceful (suggest 결과는 빈 리스트).

추가:
- lib/data/ai/{llm_service,gemma_llm_service,model_lifecycle}.dart
- lib/domain/ai/{frame_candidate,few_shot_builder,parse_response,suggest_frame}.dart
- lib/state/ai_providers.dart (aiSettings + modelAvailability + frameSuggestions)
- lib/ui/screens/settings_screen.dart (opt-in 토글 + 모델 상태 표시)
- lib/ui/widgets/frame_suggestion_dialog.dart (후보 3개 카드 + 다시 시도)
- HabitCreateScreen: "AI 제안" 버튼 (opt-in + ready 일 때만 노출)
- MetaDao.remove(key) 추가 (purge 용)

테스트 31개 신규 추가 (총 62개 통과):
- test/domain/ai/{suggest_frame, few_shot_builder, parse_response}_test.dart
- test/data/ai/model_lifecycle_test.dart (download/SHA/purge/availability)

flutter analyze 0 issue, flutter build apk --debug 통과.

Refs #215

app/lib/data/ai/gemma_llm_service.dart

@@ -0,0 +1,45 @@
+import 'llm_service.dart';
+
+/// Stub for the real `flutter_gemma` integration.
+///
+/// Wired up only after OQ-1 (exact model URL + SHA + flutter_gemma API
+/// surface) is confirmed in Developer phase. Today this throws
+/// `UnimplementedError` from every method — the rest of the system
+/// (suggestFrame, ModelLifecycle, Riverpod providers) is built against the
+/// `LlmService` abstract above and runs end-to-end with `MockLlmService`.
+///
+/// When the package is added, replace the bodies with calls into
+/// FlutterGemma.init / generateWithFunctionCalling per the package docs.
+/// Existing tests + UI hooks remain unchanged.
+class GemmaLlmService implements LlmService {
+  final String modelPath;
+
+  GemmaLlmService({required this.modelPath});
+
+  bool _loaded = false;
+
+  @override
+  bool get isLoaded => _loaded;
+
+  @override
+  Future<void> load() async {
+    throw UnimplementedError(
+      'GemmaLlmService.load: pending OQ-1 (model URL + flutter_gemma).',
+    );
+  }
+
+  @override
+  Future<void> unload() async {
+    _loaded = false;
+  }
+
+  @override
+  Future<Map<String, dynamic>> generateStructured(
+    String prompt,
+    Map<String, dynamic> schema,
+  ) async {
+    throw UnimplementedError(
+      'GemmaLlmService.generateStructured: pending OQ-1.',
+    );
+  }
+}