[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/main.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import 'data/ai/llm_service.dart';
+import 'state/ai_providers.dart';
 import 'state/providers.dart';
 import 'ui/screens/habit_list_screen.dart';
 
@@ -8,7 +10,13 @@ Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   final db = await openProductionDatabase();
   runApp(ProviderScope(
-    overrides: [appDatabaseProvider.overrideWithValue(db)],
+    overrides: [
+      appDatabaseProvider.overrideWithValue(db),
+      // OQ-1 pending: production-ready GemmaLlmService is wired here once
+      // model URL + SHA are pinned and flutter_gemma is added. Until then,
+      // MockLlmService keeps the app graceful (suggestFrame returns []).
+      llmServiceProvider.overrideWithValue(MockLlmService()),
+    ],
     child: const LifeHelperApp(),
   ));
 }