[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/widgets/frame_suggestion_dialog.dart

@@ -0,0 +1,102 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../domain/ai/frame_candidate.dart';
+import '../../domain/models/habit.dart';
+import '../../state/ai_providers.dart';
+
+/// Shows L2/L3 suggestion cards from suggestFrame. Returns the selected
+/// framed_text via `Navigator.pop(context, candidate.framedText)`.
+class FrameSuggestionDialog extends ConsumerWidget {
+  final SuggestFrameInput input;
+  const FrameSuggestionDialog({super.key, required this.input});
+
+  static Future<String?> show(
+    BuildContext context, {
+    required SuggestFrameInput input,
+  }) {
+    return showDialog<String>(
+      context: context,
+      builder: (_) => FrameSuggestionDialog(input: input),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final async = ref.watch(frameSuggestionsProvider(input));
+    return AlertDialog(
+      title: const Text('AI 제안'),
+      content: SizedBox(
+        width: 320,
+        child: async.when(
+          loading: () => const SizedBox(
+            height: 120,
+            child: Center(child: CircularProgressIndicator()),
+          ),
+          error: (e, _) => Padding(
+            padding: const EdgeInsets.all(8),
+            child: Text('AI 제안을 받지 못했습니다. 직접 입력해주세요.\n($e)'),
+          ),
+          data: (candidates) {
+            if (candidates.isEmpty) {
+              return Column(
+                mainAxisSize: MainAxisSize.min,
+                crossAxisAlignment: CrossAxisAlignment.stretch,
+                children: [
+                  const Padding(
+                    padding: EdgeInsets.all(8),
+                    child: Text(
+                      '더 구체적으로 입력해주시면 더 좋은 제안을 드릴 수 있어요.',
+                    ),
+                  ),
+                  TextButton(
+                    onPressed: () {
+                      ref.invalidate(frameSuggestionsProvider(input));
+                    },
+                    child: const Text('다시 시도'),
+                  ),
+                ],
+              );
+            }
+            return Column(
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                for (final c in candidates)
+                  _CandidateCard(
+                    candidate: c,
+                    onTap: () => Navigator.of(context).pop(c.framedText),
+                  ),
+              ],
+            );
+          },
+        ),
+      ),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.of(context).pop(),
+          child: const Text('취소'),
+        ),
+      ],
+    );
+  }
+}
+
+class _CandidateCard extends StatelessWidget {
+  final FrameCandidate candidate;
+  final VoidCallback onTap;
+  const _CandidateCard({required this.candidate, required this.onTap});
+
+  @override
+  Widget build(BuildContext context) {
+    return Card(
+      child: ListTile(
+        title: Text(candidate.framedText),
+        subtitle: Text(
+          '${candidate.level == FrameLevel.l3 ? "L3 · 정체성" : "L2 · 조건부 긍정"} '
+          '· 신뢰도 ${(candidate.confidence * 100).toInt()}%',
+        ),
+        onTap: onTap,
+      ),
+    );
+  }
+}