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/room/[id]/page.tsx

@@ -4,6 +4,7 @@ import { useEffect, useState, useCallback, useRef } from "react";
 import { useParams, useRouter } from "next/navigation";
 import TopNav from "@/components/TopNav";
 import SwipeDeck from "@/components/SwipeDeck";
+import { SwipeDeckSkeleton } from "@/components/Skeleton";
 import LeaveConfirmModal from "@/components/LeaveConfirmModal";
 import { useRoomPolling } from "@/hooks/useRoomPolling";
 import { getUserId } from "@/lib/userId";
@@ -111,12 +112,7 @@ export default function RoomPage() {
   const sceneConfig = getSceneConfig(scene);
 
   if (!ready) {
-    return (
-      <div className="flex h-dvh flex-col items-center justify-center gap-3 bg-background">
-        <div className="h-6 w-6 animate-spin rounded-full border-2 border-subtle border-t-accent" />
-        <p className="text-sm text-muted">正在加载数据...</p>
-      </div>
-    );
+    return <SwipeDeckSkeleton />;
   }
 
   return (