ui: 骨架屏替代全部页面级加载 spinner

- 新增 Skeleton 组件库：Skeleton、SkeletonCircle 基础元素 + 5 个业务骨架
  （SwipeDeck、ProfileCard、RecordItem、BlindboxRoom、BlindboxList、RoomCard）
- 替换 room、profile、blindbox 列表、blindbox 房间、invite 5 个页面的加载态
- 替换 profile 历史记录 / 收藏列表的内联加载 spinner
- 更新 project-conventions.mdc：新增 Loading States 规范，
  要求页面级和列表级加载必须使用骨架屏

.cursor/rules/project-conventions.mdc

@@ -35,6 +35,14 @@ alwaysApply: true
 - `overflow-y-auto scrollbar-none` for scrollable pages
 - Extract reusable UI patterns into shared components (modals, cards, empty states)
 
+## Loading States
+
+- **Page-level and list-level loading**: always use skeleton screens (`src/components/Skeleton.tsx`), never bare spinners
+- Skeleton shape should mimic the actual content layout (cards, list items, avatars, text lines)
+- Compose page skeletons from reusable skeleton primitives: `Skeleton`, `SkeletonCircle`, `RecordItemSkeleton`, `RoomCardSkeleton`, etc.
+- **Button-level loading** (submit, save, join): keep using inline `Loader2` spinner — skeleton screens don't apply to buttons
+- When adding a new page or async data section, always include a skeleton loading state
+
 ## API Routes
 
 - Located in `src/app/api/`