[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/ui/screens/habit_list_screen.dart

@@ -4,6 +4,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../../state/providers.dart';
 import 'check_in_screen.dart';
 import 'habit_create_screen.dart';
+import 'settings_screen.dart';
 import 'streak_screen.dart';
 
 class HabitListScreen extends ConsumerWidget {
@@ -15,7 +16,20 @@ class HabitListScreen extends ConsumerWidget {
     final habitsAsync = ref.watch(activeHabitsProvider);
 
     return Scaffold(
-      appBar: AppBar(title: const Text('습관')),
+      appBar: AppBar(
+        title: const Text('습관'),
+        actions: [
+          IconButton(
+            icon: const Icon(Icons.settings),
+            tooltip: '설정',
+            onPressed: () {
+              Navigator.of(context).push(MaterialPageRoute(
+                builder: (_) => const SettingsScreen(),
+              ));
+            },
+          ),
+        ],
+      ),
       body: boot.when(
         loading: () => const Center(child: CircularProgressIndicator()),
         error: (e, st) => Center(child: Text('초기화 실패: $e')),