[Developer] #215 AC2/AC4/AC6 fixes after QA reject

QA 1차 (커밋 6ab4c0d 검증) 에서 3건 AC 미충족 → 03-Developer 반려.
본 커밋은 그 3건을 해결한다.

AC6 — _AiSuggestButton 가시성 분기 분리:
- optIn=false → 숨김 유지
- optIn=true && !ready → 노출 + disabled + Tooltip("AI 도움을 먼저 켜주세요")
- optIn=true && ready → enabled
AC5 보완: 후보의 c.level 직접 전달 (이전 "나는" 휴리스틱 제거).
FrameSuggestionDialog.show 반환 타입 String? → FrameCandidate?.

AC4 — L2:2 + L3:1 분포 강제:
- few_shot prompt 에 "정확히 L2 2개 + L3 1개" 명시
- suggestFrame 결과를 _shapeDistribution(l2Quota=2, l3Quota=1) 로 후처리
- 부족분은 패딩 X (graceful: UI 가 더 적은 카드만 표시)

AC2 — 다운로드 진행률 + 일시정지/재개 UI:
- ModelDownloadController (StateNotifier<DownloadProgress?>)
  · start() / pause() / resume() / cancel()
  · pause() 는 subscription 만 cancel, .tmp + meta_kv 유지 → resume 시 Range header 로 이어받음
- AiSettingsController.setOptIn(true) → controller.start() 자동 호출
- SettingsScreen 에 _DownloadProgressTile 추가
  · LinearProgressIndicator + bytes/total + 일시정지/재개/다시 시도 토글

회귀 테스트 9건 신규:
- test/ui/ai_suggest_button_visibility_test.dart (4): AC6 4상태 (hidden / disabled+tooltip × missing/downloading / enabled)
- test/state/model_download_controller_test.dart (3): opt-in→start, pause→paused, cancel→idle
- test/domain/ai/suggest_frame_test.dart (+3): AC4 분포 케이스 3개 (기존 take(3) 테스트 대체)

검증:
- flutter analyze → No issues found
- flutter test → 71 tests pass (62 → 71, +9 신규)
- flutter build apk --debug → 성공 (8.8s)

OQ-1 (모델 URL+SHA) 미해결 유지. MockLlmService 기본 주입 + placeholder URL 다운로드는 여전히 실패하지만, UI/스트림 wiring 은 모두 검증됨.

Refs #215

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/test/ui/ai_suggest_button_visibility_test.dart

@@ -0,0 +1,121 @@
+// AC6 regression: button is hidden when opt-in OFF, but visible-but-disabled
+// with a Tooltip when opt-in ON and model is not ready.
+//
+// The QA reject (2026-06-12) caught that the prior implementation rendered
+// `SizedBox.shrink()` in BOTH cases, eliminating discoverability for opted-in
+// users still downloading.
+import 'package:drift/native.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:life_helper/data/ai/llm_service.dart';
+import 'package:life_helper/data/ai/model_lifecycle.dart';
+import 'package:life_helper/data/db/app_database.dart';
+import 'package:life_helper/state/ai_providers.dart';
+import 'package:life_helper/state/providers.dart';
+import 'package:life_helper/ui/screens/habit_create_screen.dart';
+
+Future<AppDatabase> _memDb() async {
+  return AppDatabase(NativeDatabase.memory());
+}
+
+ProviderContainer _container({
+  required AppDatabase db,
+  required bool optIn,
+  required ModelAvailability availability,
+}) {
+  return ProviderContainer(overrides: [
+    appDatabaseProvider.overrideWithValue(db),
+    llmServiceProvider.overrideWithValue(MockLlmService()),
+    aiSettingsProvider.overrideWith((_) async => optIn),
+    modelAvailabilityProvider.overrideWith((_) async => availability),
+  ]);
+}
+
+Widget _wrap(ProviderContainer container) {
+  return UncontrolledProviderScope(
+    container: container,
+    child: const MaterialApp(home: Scaffold(body: HabitCreateScreen())),
+  );
+}
+
+void main() {
+  testWidgets('AC6: optIn=false → button hidden entirely', (tester) async {
+    final db = await _memDb();
+    addTearDown(db.close);
+    final c = _container(
+      db: db,
+      optIn: false,
+      availability: ModelAvailability.ready,
+    );
+    addTearDown(c.dispose);
+    await tester.pumpWidget(_wrap(c));
+    await tester.pumpAndSettle();
+
+    expect(find.text('AI 제안'), findsNothing);
+    expect(find.byTooltip('AI 도움을 먼저 켜주세요'), findsNothing);
+  });
+
+  testWidgets(
+      'AC6: optIn=true, availability=missing → button visible, disabled, tooltip',
+      (tester) async {
+    final db = await _memDb();
+    addTearDown(db.close);
+    final c = _container(
+      db: db,
+      optIn: true,
+      availability: ModelAvailability.missing,
+    );
+    addTearDown(c.dispose);
+    await tester.pumpWidget(_wrap(c));
+    await tester.pumpAndSettle();
+
+    // Visible
+    expect(find.text('AI 제안'), findsOneWidget);
+    // Wrapped in tooltip with the AC6 exact message
+    expect(find.byTooltip('AI 도움을 먼저 켜주세요'), findsOneWidget);
+    // Disabled
+    final TextButton btn = tester.widget(find.byType(TextButton).first);
+    expect(btn.onPressed, isNull);
+  });
+
+  testWidgets(
+      'AC6: optIn=true, availability=downloading → button visible+disabled',
+      (tester) async {
+    final db = await _memDb();
+    addTearDown(db.close);
+    final c = _container(
+      db: db,
+      optIn: true,
+      availability: ModelAvailability.downloading,
+    );
+    addTearDown(c.dispose);
+    await tester.pumpWidget(_wrap(c));
+    await tester.pumpAndSettle();
+
+    expect(find.text('AI 제안'), findsOneWidget);
+    expect(find.byTooltip('AI 도움을 먼저 켜주세요'), findsOneWidget);
+    final TextButton btn = tester.widget(find.byType(TextButton).first);
+    expect(btn.onPressed, isNull);
+  });
+
+  testWidgets(
+      'AC6: optIn=true, availability=ready → button enabled, no tooltip wrapper',
+      (tester) async {
+    final db = await _memDb();
+    addTearDown(db.close);
+    final c = _container(
+      db: db,
+      optIn: true,
+      availability: ModelAvailability.ready,
+    );
+    addTearDown(c.dispose);
+    await tester.pumpWidget(_wrap(c));
+    await tester.pumpAndSettle();
+
+    expect(find.text('AI 제안'), findsOneWidget);
+    expect(find.byTooltip('AI 도움을 먼저 켜주세요'), findsNothing);
+    final TextButton btn = tester.widget(find.byType(TextButton).first);
+    expect(btn.onPressed, isNotNull);
+  });
+}