[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/ui/screens/settings_screen.dart

@@ -43,6 +43,8 @@ class _AiSection extends ConsumerWidget {
   Widget build(BuildContext context, WidgetRef ref) {
     final settings = ref.watch(aiSettingsProvider);
     final availability = ref.watch(modelAvailabilityProvider);
+    final download = ref.watch(modelDownloadControllerProvider);
+    final optIn = settings.maybeWhen(data: (v) => v, orElse: () => false);
     return Column(
       crossAxisAlignment: CrossAxisAlignment.stretch,
       children: [
@@ -51,10 +53,7 @@ class _AiSection extends ConsumerWidget {
           subtitle: const Text(
             'Gemma 4 E2B 모델 ≈ 1.5GB. 모든 처리는 단말에서 일어납니다.',
           ),
-          value: settings.maybeWhen(
-            data: (v) => v,
-            orElse: () => false,
-          ),
+          value: optIn,
           onChanged: (v) async {
             if (v) {
               final ok = await _confirmOptIn(context);
@@ -81,6 +80,8 @@ class _AiSection extends ConsumerWidget {
             subtitle: Text(_describe(a)),
           ),
         ),
+        if (optIn && download != null)
+          _DownloadProgressTile(progress: download, ref: ref),
         const Padding(
           padding: EdgeInsets.fromLTRB(16, 8, 16, 16),
           child: Text(
@@ -159,3 +160,90 @@ class _AiSection extends ConsumerWidget {
     );
   }
 }
+
+/// AC2: shows download progress + pause/resume/restart controls.
+class _DownloadProgressTile extends StatelessWidget {
+  final DownloadProgress progress;
+  final WidgetRef ref;
+  const _DownloadProgressTile({required this.progress, required this.ref});
+
+  @override
+  Widget build(BuildContext context) {
+    final controller = ref.read(modelDownloadControllerProvider.notifier);
+    final pct = progress.totalBytes > 0
+        ? (progress.bytesReceived / progress.totalBytes).clamp(0.0, 1.0)
+        : null;
+    final pctText = pct != null
+        ? '${(pct * 100).toStringAsFixed(0)}%'
+        : '계산 중...';
+    final size =
+        '${_fmtBytes(progress.bytesReceived)}'
+        '${progress.totalBytes > 0 ? " / ${_fmtBytes(progress.totalBytes)}" : ""}';
+
+    Widget controls;
+    switch (progress.state) {
+      case DownloadState.downloading:
+        controls = TextButton.icon(
+          icon: const Icon(Icons.pause),
+          label: const Text('일시정지'),
+          onPressed: controller.pause,
+        );
+        break;
+      case DownloadState.paused:
+        controls = TextButton.icon(
+          icon: const Icon(Icons.play_arrow),
+          label: const Text('재개'),
+          onPressed: controller.resume,
+        );
+        break;
+      case DownloadState.failed:
+        controls = TextButton.icon(
+          icon: const Icon(Icons.refresh),
+          label: const Text('다시 시도'),
+          onPressed: controller.resume,
+        );
+        break;
+      case DownloadState.completed:
+        controls = const Text('완료', style: TextStyle(color: Colors.green));
+        break;
+      case DownloadState.idle:
+        controls = const SizedBox.shrink();
+        break;
+    }
+
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(16, 0, 16, 8),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          Row(
+            children: [
+              Expanded(child: Text('$size · $pctText')),
+              controls,
+            ],
+          ),
+          const SizedBox(height: 4),
+          LinearProgressIndicator(value: pct),
+          if (progress.state == DownloadState.failed &&
+              progress.errorMessage != null)
+            Padding(
+              padding: const EdgeInsets.only(top: 4),
+              child: Text(
+                '에러: ${progress.errorMessage}',
+                style: const TextStyle(fontSize: 12, color: Colors.red),
+              ),
+            ),
+        ],
+      ),
+    );
+  }
+
+  String _fmtBytes(int bytes) {
+    if (bytes < 1024) return '$bytes B';
+    if (bytes < 1024 * 1024) return '${(bytes / 1024).toStringAsFixed(1)} KB';
+    if (bytes < 1024 * 1024 * 1024) {
+      return '${(bytes / (1024 * 1024)).toStringAsFixed(1)} MB';
+    }
+    return '${(bytes / (1024 * 1024 * 1024)).toStringAsFixed(2)} GB';
+  }
+}