[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/test/domain/ai/parse_response_test.dart

@@ -0,0 +1,90 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:life_helper/domain/ai/parse_response.dart';
+import 'package:life_helper/domain/models/habit.dart';
+
+void main() {
+  test('parses 3 valid candidates', () {
+    final r = parseFrameCandidates({
+      'candidates': [
+        {'level': 'L2', 'framed_text': '저녁엔 무알콜 마시기', 'confidence': 0.9},
+        {'level': 'L3', 'framed_text': '나는 맑은 정신의 사람이다'},
+        {'level': 'L2', 'framed_text': '주중엔 운동 우선'},
+      ],
+    });
+    expect(r, hasLength(3));
+    expect(r[0].level, FrameLevel.l2);
+    expect(r[0].confidence, 0.9);
+    expect(r[1].level, FrameLevel.l3);
+    expect(r[1].confidence, 0.5); // default
+  });
+
+  test('candidates missing → FormatException', () {
+    expect(
+      () => parseFrameCandidates({'foo': 'bar'}),
+      throwsA(isA<FormatException>()),
+    );
+  });
+
+  test('candidates not list → FormatException', () {
+    expect(
+      () => parseFrameCandidates({'candidates': 'oops'}),
+      throwsA(isA<FormatException>()),
+    );
+  });
+
+  test('empty candidates → empty list (no throw)', () {
+    final r = parseFrameCandidates({'candidates': []});
+    expect(r, isEmpty);
+  });
+
+  test('skips unknown level + length-violating items', () {
+    final tooLong = 'a' * 121;
+    final r = parseFrameCandidates({
+      'candidates': [
+        {'level': 'L99', 'framed_text': '?'}, // skipped
+        {'level': 'L2', 'framed_text': tooLong}, // skipped
+        {'level': 'L3', 'framed_text': '   '}, // skipped (empty after trim)
+        {'level': 'L2', 'framed_text': '유효한 후보'},
+      ],
+    });
+    expect(r, hasLength(1));
+    expect(r.first.framedText, '유효한 후보');
+  });
+
+  test('confidence clamps and falls back to 0.5', () {
+    final r = parseFrameCandidates({
+      'candidates': [
+        {'level': 'L2', 'framed_text': 'a', 'confidence': -0.4},
+        {'level': 'L2', 'framed_text': 'b', 'confidence': 2.5},
+        {'level': 'L2', 'framed_text': 'c', 'confidence': 'not-a-number'},
+      ],
+    });
+    expect(r[0].confidence, 0.0);
+    expect(r[1].confidence, 1.0);
+    expect(r[2].confidence, 0.5);
+  });
+
+  test('keeps L0/L1 candidates (filtering is suggestFrame responsibility)', () {
+    final r = parseFrameCandidates({
+      'candidates': [
+        {'level': 'L0', 'framed_text': '술 끊기'},
+        {'level': 'L2', 'framed_text': '무알콜'},
+      ],
+    });
+    expect(r, hasLength(2));
+    expect(r[0].level, FrameLevel.l0);
+  });
+
+  test('source_pattern_id preserved when present', () {
+    final r = parseFrameCandidates({
+      'candidates': [
+        {
+          'level': 'L2',
+          'framed_text': 'foo',
+          'source_pattern_id': 'fp_alcohol'
+        },
+      ],
+    });
+    expect(r.single.sourcePatternId, 'fp_alcohol');
+  });
+}