[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_create_screen.dart

@@ -3,10 +3,14 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../core/constants.dart';
 import '../../core/time.dart';
+import '../../data/ai/model_lifecycle.dart';
 import '../../data/db/daos/habit_dao.dart';
+import '../../domain/ai/frame_candidate.dart';
 import '../../domain/models/habit.dart';
 import '../../domain/rules/active_habit_quota.dart';
+import '../../state/ai_providers.dart';
 import '../../state/providers.dart';
+import '../widgets/frame_suggestion_dialog.dart';
 
 class HabitCreateScreen extends ConsumerStatefulWidget {
   const HabitCreateScreen({super.key});
@@ -118,7 +122,14 @@ class _HabitCreateScreenState extends ConsumerState<HabitCreateScreen> {
                 validator: (v) =>
                     (v == null || v.trim().isEmpty) ? '프레임 문구를 입력하세요' : null,
               ),
-              const SizedBox(height: 24),
+              const SizedBox(height: 8),
+              _AiSuggestButton(
+                titleCtrl: _titleCtrl,
+                framedCtrl: _framedCtrl,
+                habitType: _type,
+                onSelectLevel: (level) => setState(() => _level = level),
+              ),
+              const SizedBox(height: 16),
               FilledButton(
                 onPressed: _saving ? null : _save,
                 child: Text(_saving ? '저장 중...' : '저장'),
@@ -135,3 +146,63 @@ String _ymd(DateTime d) =>
     '${d.year.toString().padLeft(4, '0')}-'
     '${d.month.toString().padLeft(2, '0')}-'
     '${d.day.toString().padLeft(2, '0')}';
+
+/// "AI 제안" button — only visible when opt-in is ON and model is ready.
+/// Graceful: hidden otherwise (AC-5, AC-9).
+class _AiSuggestButton extends ConsumerWidget {
+  final TextEditingController titleCtrl;
+  final TextEditingController framedCtrl;
+  final HabitType habitType;
+  final ValueChanged<FrameLevel> onSelectLevel;
+  const _AiSuggestButton({
+    required this.titleCtrl,
+    required this.framedCtrl,
+    required this.habitType,
+    required this.onSelectLevel,
+  });
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final settings = ref.watch(aiSettingsProvider);
+    final availability = ref.watch(modelAvailabilityProvider);
+    final visible = settings.maybeWhen(
+          data: (v) => v,
+          orElse: () => false,
+        ) &&
+        availability.maybeWhen(
+          data: (a) => a == ModelAvailability.ready,
+          orElse: () => false,
+        );
+    if (!visible) return const SizedBox.shrink();
+    return Align(
+      alignment: Alignment.centerRight,
+      child: TextButton.icon(
+        icon: const Icon(Icons.auto_awesome),
+        label: const Text('AI 제안'),
+        onPressed: () async {
+          final raw = titleCtrl.text.trim();
+          if (raw.isEmpty) {
+            ScaffoldMessenger.of(context).showSnackBar(
+              const SnackBar(content: Text('먼저 제목을 입력해주세요')),
+            );
+            return;
+          }
+          final picked = await FrameSuggestionDialog.show(
+            context,
+            input: SuggestFrameInput(
+              rawText: raw,
+              habitType: habitType,
+            ),
+          );
+          if (picked != null) {
+            framedCtrl.text = picked;
+            // Heuristic: identity-style ("나는") → L3, else L2.
+            onSelectLevel(
+              picked.startsWith('나는') ? FrameLevel.l3 : FrameLevel.l2,
+            );
+          }
+        },
+      ),
+    );
+  }
+}