[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/widgets/reference_expand_card.dart

@@ -0,0 +1,71 @@
+import 'package:flutter/material.dart';
+
+import '../../data/db/app_database.dart';
+
+/// reference 1건을 펼치기 카드로 표시.
+///
+/// 본 이슈 (#226) 에선 url 표시만 (탭 시 launcher 호출 X — #FF1 이후).
+class ReferenceExpandCard extends StatelessWidget {
+  const ReferenceExpandCard({super.key, required this.reference});
+
+  final ReferenceRow reference;
+
+  @override
+  Widget build(BuildContext context) {
+    final kindLabel = switch (reference.kind) {
+      'paper' => '논문',
+      'podcast_episode' => '팟캐스트',
+      'book' => '서적',
+      'url' => '웹',
+      'korean_explainer' => '한국어 해설',
+      _ => reference.kind,
+    };
+    return Card(
+      child: ExpansionTile(
+        title: Text(
+          reference.title,
+          maxLines: 2,
+          overflow: TextOverflow.ellipsis,
+        ),
+        subtitle: Text(kindLabel,
+            style: Theme.of(context).textTheme.bodySmall),
+        children: [
+          Padding(
+            padding: const EdgeInsets.fromLTRB(16, 0, 16, 12),
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: [
+                if (reference.year != null)
+                  _row(context, '연도', reference.year.toString()),
+                if (reference.journal != null)
+                  _row(context, '저널', reference.journal!),
+                if (reference.publisher != null)
+                  _row(context, '출판', reference.publisher!),
+                if (reference.episodeNumber != null)
+                  _row(context, '에피소드',
+                      reference.episodeNumber.toString()),
+                if (reference.doi != null) _row(context, 'DOI', reference.doi!),
+                if (reference.url != null) _row(context, 'URL', reference.url!),
+                if (reference.note != null)
+                  _row(context, '메모', reference.note!),
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+
+  Widget _row(BuildContext context, String label, String value) {
+    return Padding(
+      padding: const EdgeInsets.only(bottom: 2),
+      child: Text.rich(TextSpan(children: [
+        TextSpan(
+          text: '$label: ',
+          style: const TextStyle(fontWeight: FontWeight.bold),
+        ),
+        TextSpan(text: value),
+      ]), style: Theme.of(context).textTheme.bodySmall),
+    );
+  }
+}