[03-Developer] #226 Catalog Gallery 구현

- Drift schema v2: Protocols.category CHECK 6→7 (light_circadian/sleep/movement/
  nutrition/focus_cognition/recovery_stress/emotion_relationship). schemaVersion
  1→2 + onUpgrade migrateV1ToV2 (DROP+CREATE+reseed flag 클리어).
- protocols.json 34 항목 v2 재분류 (1차 효과 기준). emotion_relationship 0 매핑.
- 도메인: DisplayCategory enum (8) + CatalogItem sealed (Protocol/Break/Diet).
- 데이터: CatalogRepository.all/byId/referencesByIds (3 source 통합).
- 상태: catalog_providers.dart (catalogItems / groupedByCategory / refsByIds).
- UI: ProtocolGalleryScreen (카테고리 칩 + 카드 그리드) + ProtocolPreviewScreen
  (모든 필드 + reference 펼치기 + "내 습관으로" disabled placeholder) +
  CatalogCard / CategoryChipRow / ReferenceExpandCard. HabitListScreen 빈
  상태 CTA + AppBar 액션.
- 테스트: migration_v1_to_v2 3건 + display_category 5건 + catalog_repository
  9건 + gallery widget 3건 + preview widget 3건 = 23 신규. 기존 88 회귀 0,
  flutter analyze 0 issues. 110 passed / 1 skipped.

설계서: docs/design/226-catalog-gallery/{README, fn-catalog_repository,
fn-migration_v1_to_v2}.md + ADR-0004.

Refs #226

app/lib/ui/screens/habit_list_screen.dart

@@ -4,6 +4,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../../state/providers.dart';
 import 'check_in_screen.dart';
 import 'habit_create_screen.dart';
+import 'protocol_gallery_screen.dart';
 import 'settings_screen.dart';
 import 'streak_screen.dart';
 
@@ -19,6 +20,11 @@ class HabitListScreen extends ConsumerWidget {
       appBar: AppBar(
         title: const Text('습관'),
         actions: [
+          IconButton(
+            icon: const Icon(Icons.search),
+            tooltip: '카탈로그 탐색',
+            onPressed: () => _openGallery(context),
+          ),
           IconButton(
             icon: const Icon(Icons.settings),
             tooltip: '설정',
@@ -38,8 +44,25 @@ class HabitListScreen extends ConsumerWidget {
           error: (e, st) => Center(child: Text('로드 실패: $e')),
           data: (habits) {
             if (habits.isEmpty) {
-              return const Center(
-                child: Text('아직 습관이 없습니다. + 버튼으로 추가하세요.'),
+              return Center(
+                child: Column(
+                  mainAxisSize: MainAxisSize.min,
+                  children: [
+                    const Padding(
+                      padding: EdgeInsets.symmetric(horizontal: 24),
+                      child: Text(
+                        '아직 습관이 없습니다.\n+ 버튼으로 추가하거나, 카탈로그에서 골라보세요.',
+                        textAlign: TextAlign.center,
+                      ),
+                    ),
+                    const SizedBox(height: 16),
+                    FilledButton.icon(
+                      onPressed: () => _openGallery(context),
+                      icon: const Icon(Icons.search),
+                      label: const Text('🔍 카탈로그 탐색'),
+                    ),
+                  ],
+                ),
               );
             }
             return ListView.separated(
@@ -83,4 +106,10 @@ class HabitListScreen extends ConsumerWidget {
       ),
     );
   }
+
+  void _openGallery(BuildContext context) {
+    Navigator.of(context).push(MaterialPageRoute(
+      builder: (_) => const ProtocolGalleryScreen(),
+    ));
+  }
 }