[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/habit_create_screen.dart

@@ -147,8 +147,10 @@ String _ymd(DateTime d) =>
     '${d.month.toString().padLeft(2, '0')}-'
     '${d.day.toString().padLeft(2, '0')}';
 
-/// "AI 제안" button — only visible when opt-in is ON and model is ready.
-/// Graceful: hidden otherwise (AC-5, AC-9).
+/// "AI 제안" button — three states (AC3, AC6, AC7):
+/// - opt-in OFF → hidden (discoverability gated by Settings)
+/// - opt-in ON && model not ready → visible but DISABLED + tooltip
+/// - opt-in ON && model ready → enabled
 class _AiSuggestButton extends ConsumerWidget {
   final TextEditingController titleCtrl;
   final TextEditingController framedCtrl;
@@ -163,46 +165,53 @@ class _AiSuggestButton extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final settings = ref.watch(aiSettingsProvider);
-    final availability = ref.watch(modelAvailabilityProvider);
-    final visible = settings.maybeWhen(
+    final optIn = ref.watch(aiSettingsProvider).maybeWhen(
           data: (v) => v,
           orElse: () => false,
-        ) &&
-        availability.maybeWhen(
+        );
+    if (!optIn) return const SizedBox.shrink();
+
+    final ready = ref.watch(modelAvailabilityProvider).maybeWhen(
           data: (a) => a == ModelAvailability.ready,
           orElse: () => false,
         );
-    if (!visible) return const SizedBox.shrink();
+
+    final button = TextButton.icon(
+      icon: const Icon(Icons.auto_awesome),
+      label: const Text('AI 제안'),
+      onPressed: ready
+          ? () async {
+              final raw = titleCtrl.text.trim();
+              if (raw.isEmpty) {
+                ScaffoldMessenger.of(context).showSnackBar(
+                  const SnackBar(content: Text('먼저 제목을 입력해주세요')),
+                );
+                return;
+              }
+              final picked = await FrameSuggestionDialog.show(
+                context,
+                input: SuggestFrameInput(
+                  rawText: raw,
+                  habitType: habitType,
+                ),
+              );
+              if (picked != null) {
+                framedCtrl.text = picked.framedText;
+                // AC5: use the candidate's explicit level, not a heuristic.
+                onSelectLevel(picked.level);
+              }
+            }
+          : null,
+    );
+
     return Align(
       alignment: Alignment.centerRight,
-      child: TextButton.icon(
-        icon: const Icon(Icons.auto_awesome),
-        label: const Text('AI 제안'),
-        onPressed: () async {
-          final raw = titleCtrl.text.trim();
-          if (raw.isEmpty) {
-            ScaffoldMessenger.of(context).showSnackBar(
-              const SnackBar(content: Text('먼저 제목을 입력해주세요')),
-            );
-            return;
-          }
-          final picked = await FrameSuggestionDialog.show(
-            context,
-            input: SuggestFrameInput(
-              rawText: raw,
-              habitType: habitType,
+      child: ready
+          ? button
+          : Tooltip(
+              message: 'AI 도움을 먼저 켜주세요',
+              child: button,
             ),
-          );
-          if (picked != null) {
-            framedCtrl.text = picked;
-            // Heuristic: identity-style ("나는") → L3, else L2.
-            onSelectLevel(
-              picked.startsWith('나는') ? FrameLevel.l3 : FrameLevel.l2,
-            );
-          }
-        },
-      ),
     );
   }
 }