[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/lib/state/ai_providers.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../data/ai/llm_service.dart';
@@ -33,6 +35,8 @@ final aiSettingsProvider = FutureProvider<bool>((ref) async {
 });
 
 /// Toggles opt-in. On opt-out, purges model file via [ModelLifecycle.purge].
+/// On opt-in, kicks off `ModelDownloadController.start()` so AC2 (progress UI)
+/// has a stream to subscribe to.
 class AiSettingsController {
   AiSettingsController(this.ref);
   final Ref ref;
@@ -43,8 +47,13 @@ class AiSettingsController {
       await meta.put(AiMetaKeys.optIn, 'true');
       ref.invalidate(aiSettingsProvider);
       ref.invalidate(modelAvailabilityProvider);
+      // AC2: opt-in triggers download stream so Settings UI can render
+      // progress + pause/resume. Fire-and-forget; controller emits states.
+      ref.read(modelDownloadControllerProvider.notifier).start();
       return 0;
     }
+    // opt-out: cancel any in-flight download, then purge.
+    ref.read(modelDownloadControllerProvider.notifier).cancel();
     final freed = await ref.read(modelLifecycleProvider).purge();
     await meta.put(AiMetaKeys.optIn, 'false');
     ref.invalidate(aiSettingsProvider);
@@ -57,6 +66,72 @@ final aiSettingsControllerProvider = Provider<AiSettingsController>((ref) {
   return AiSettingsController(ref);
 });
 
+/// AC2: streams DownloadProgress + supports pause/resume/cancel.
+/// State `null` means idle (no active subscription).
+class ModelDownloadController extends StateNotifier<DownloadProgress?> {
+  ModelDownloadController(this.ref) : super(null);
+  final Ref ref;
+  StreamSubscription<DownloadProgress>? _sub;
+
+  void start() {
+    cancel();
+    final lc = ref.read(modelLifecycleProvider);
+    _sub = lc.download().listen(
+      (p) {
+        state = p;
+        if (p.state == DownloadState.completed) {
+          ref.invalidate(modelAvailabilityProvider);
+        }
+      },
+      onError: (Object e) {
+        state = DownloadProgress(
+          bytesReceived: state?.bytesReceived ?? 0,
+          totalBytes: state?.totalBytes ?? -1,
+          state: DownloadState.failed,
+          errorMessage: e.toString(),
+        );
+      },
+      onDone: () {
+        _sub = null;
+      },
+    );
+  }
+
+  /// Pauses by cancelling the subscription. .tmp file + meta_kv preserved so
+  /// `start()` resumes via HTTP Range header.
+  void pause() {
+    _sub?.cancel();
+    _sub = null;
+    final cur = state;
+    if (cur != null && cur.state != DownloadState.completed) {
+      state = DownloadProgress(
+        bytesReceived: cur.bytesReceived,
+        totalBytes: cur.totalBytes,
+        state: DownloadState.paused,
+      );
+    }
+  }
+
+  void resume() => start();
+
+  void cancel() {
+    _sub?.cancel();
+    _sub = null;
+    state = null;
+  }
+
+  @override
+  void dispose() {
+    _sub?.cancel();
+    super.dispose();
+  }
+}
+
+final modelDownloadControllerProvider =
+    StateNotifierProvider<ModelDownloadController, DownloadProgress?>(
+  (ref) => ModelDownloadController(ref),
+);
+
 final modelAvailabilityProvider =
     FutureProvider<ModelAvailability>((ref) async {
   final lc = ref.watch(modelLifecycleProvider);