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

src/app/blindbox/[code]/page.tsx

@@ -20,6 +20,7 @@ import {
 import confetti from "canvas-confetti";
 import { getCachedProfile, isRegistered } from "@/lib/userId";
 import ShareCardModal from "@/components/ShareCardModal";
+import { BlindboxRoomSkeleton } from "@/components/Skeleton";
 import type { UserProfile } from "@/types";
 
 interface RoomInfo {
@@ -285,11 +286,7 @@ export default function BlindboxRoomPage() {
   };
 
   if (pageLoading) {
-    return (
-      <div className="flex min-h-dvh items-center justify-center bg-background">
-        <Loader2 size={24} className="animate-spin text-purple-400" />
-      </div>
-    );
+    return <BlindboxRoomSkeleton />;
   }
 
   if (!room) return null;