feat(mobile): UI-01 album cards, photo grid and full viewer

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/mobile/test/helpers/fakes.dart

@@ -262,6 +262,9 @@ Map<String, dynamic> adminPhotoJson({
   String visibility = 'published',
   String processingState = 'ready',
   int bytes = 123456,
+  String uploaderAccount = 'staff02',
+  String uploaderName = 'Staff Two',
+  Map<String, dynamic>? preview,
 }) => <String, dynamic>{
   'photoId': photoId,
   'albumId': albumId,
@@ -278,10 +281,25 @@ Map<String, dynamic> adminPhotoJson({
   'likeCount': 0,
   'uploader': <String, dynamic>{
     'userId': 'u-2',
-    'account': 'staff02',
-    'displayName': 'Staff Two',
+    'account': uploaderAccount,
+    'displayName': uploaderName,
+  },
+  'preview': preview,
+};
+
+/// 契约 `PhotoPreview` 响应体（thumb/browse 签名 URL）。
+Map<String, dynamic> photoPreviewJson({
+  String thumbUrl = 'http://test.local/thumb.jpg',
+  String browseUrl = 'http://test.local/browse.jpg',
+}) => <String, dynamic>{
+  'thumb': {
+    'url': thumbUrl,
+    'expiresAt': '2026-09-22T00:00:00Z',
+  },
+  'browse': {
+    'url': browseUrl,
+    'expiresAt': '2026-09-22T00:00:00Z',
   },
-  'preview': null,
 };
 
 /// 契约游标分页响应体（hasMore 与 nextCursor 成对）。