[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/state/ai_providers.dart

@@ -0,0 +1,113 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../data/ai/llm_service.dart';
+import '../data/ai/model_lifecycle.dart';
+import '../data/db/app_database.dart' as drift;
+import '../domain/ai/frame_candidate.dart';
+import '../domain/ai/suggest_frame.dart';
+import '../domain/models/frame_pattern.dart';
+import 'providers.dart';
+
+/// Default config for the on-device Gemma model (#215).
+/// OQ-1: URL + SHA-256 pinned in Developer phase. Until then, downloads are
+/// disabled (AI toggle is gated behind these constants being real).
+const _kModelUrlPlaceholder =
+    'https://example.invalid/gemma4-e2b-q4.bin'; // OQ-1
+const _kModelShaPlaceholder = 'PENDING_OQ_1';
+
+final modelLifecycleProvider = Provider<ModelLifecycle>((ref) {
+  return ModelLifecycle(
+    meta: ref.watch(metaDaoProvider),
+    config: ModelConfig(
+      url: Uri.parse(_kModelUrlPlaceholder),
+      expectedSha256: _kModelShaPlaceholder,
+    ),
+  );
+});
+
+/// Read-only opt-in state. Default OFF; persisted in `meta_kv`.
+final aiSettingsProvider = FutureProvider<bool>((ref) async {
+  final meta = ref.watch(metaDaoProvider);
+  final v = await meta.find(AiMetaKeys.optIn);
+  return v == 'true';
+});
+
+/// Toggles opt-in. On opt-out, purges model file via [ModelLifecycle.purge].
+class AiSettingsController {
+  AiSettingsController(this.ref);
+  final Ref ref;
+
+  Future<int> setOptIn(bool value) async {
+    final meta = ref.read(metaDaoProvider);
+    if (value) {
+      await meta.put(AiMetaKeys.optIn, 'true');
+      ref.invalidate(aiSettingsProvider);
+      ref.invalidate(modelAvailabilityProvider);
+      return 0;
+    }
+    final freed = await ref.read(modelLifecycleProvider).purge();
+    await meta.put(AiMetaKeys.optIn, 'false');
+    ref.invalidate(aiSettingsProvider);
+    ref.invalidate(modelAvailabilityProvider);
+    return freed;
+  }
+}
+
+final aiSettingsControllerProvider = Provider<AiSettingsController>((ref) {
+  return AiSettingsController(ref);
+});
+
+final modelAvailabilityProvider =
+    FutureProvider<ModelAvailability>((ref) async {
+  final lc = ref.watch(modelLifecycleProvider);
+  return lc.checkAvailability();
+});
+
+/// Loads FramePatterns from DB and converts to domain models.
+final framePatternsProvider = FutureProvider<List<FramePatternModel>>(
+  (ref) async {
+    final db = ref.watch(appDatabaseProvider);
+    final rows = await db.select(db.framePatterns).get();
+    return rows.map(_toDomain).toList(growable: false);
+  },
+);
+
+FramePatternModel _toDomain(drift.FramePattern r) => FramePatternModel(
+      id: r.id,
+      domain: r.domain,
+      avoidanceKeyword: r.avoidanceKeyword,
+      l0Example: r.l0Example,
+      l1SimpleReplace: r.l1SimpleReplace,
+      l2Suggestion: r.l2Suggestion,
+      l3Identity: r.l3Identity,
+    );
+
+/// Singleton LLM service for the app. v1 starts unloaded; first
+/// [suggestFrame] triggers `.load()` via the dialog. Override in tests with
+/// `MockLlmService`.
+final llmServiceProvider = Provider<LlmService>((ref) {
+  throw UnimplementedError(
+    'llmServiceProvider must be overridden (Mock in tests, '
+    'GemmaLlmService after OQ-1 in production).',
+  );
+});
+
+/// `family` param wraps a SuggestFrameInput. Loads model lazily before
+/// calling suggestFrame.
+final frameSuggestionsProvider = FutureProvider.autoDispose
+    .family<List<FrameCandidate>, SuggestFrameInput>((ref, input) async {
+  final llm = ref.watch(llmServiceProvider);
+  final patterns = await ref.watch(framePatternsProvider.future);
+  if (!llm.isLoaded) {
+    try {
+      await llm.load();
+    } catch (_) {
+      return const [];
+    }
+  }
+  return suggestFrame(
+    input,
+    llm: llm,
+    framePatterns: patterns,
+  );
+});