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/invite/[id]/page.tsx

@@ -13,6 +13,7 @@ import {
   Coffee,
 } from "lucide-react";
 import { getUserId } from "@/lib/userId";
+import { Skeleton, SkeletonCircle } from "@/components/Skeleton";
 import { getSceneConfig } from "@/lib/sceneConfig";
 import type { SceneType } from "@/types";
 
@@ -61,8 +62,23 @@ export default function InvitePage() {
 
   if (status === "loading") {
     return (
-      <div className="flex min-h-dvh flex-col items-center justify-center bg-background">
-        <div className="h-6 w-6 animate-spin rounded-full border-2 border-subtle border-t-accent" />
+      <div className="flex min-h-dvh flex-col items-center justify-center bg-background px-6">
+        <Skeleton className="h-16 w-16 rounded-2xl" />
+        <Skeleton className="mt-5 h-8 w-40" />
+        <Skeleton className="mt-2 h-4 w-24" />
+        <div className="mt-8 w-full max-w-xs rounded-2xl bg-surface px-6 py-5 ring-1 ring-border">
+          <div className="flex flex-col items-center gap-3">
+            <Skeleton className="h-5 w-44" />
+            <Skeleton className="h-7 w-24 rounded-full" />
+            <Skeleton className="h-4 w-32" />
+          </div>
+        </div>
+        <div className="mt-6 flex gap-3">
+          <SkeletonCircle className="h-10 w-10" />
+          <SkeletonCircle className="h-10 w-10" />
+          <SkeletonCircle className="h-10 w-10" />
+        </div>
+        <Skeleton className="mt-8 h-12 w-full max-w-xs rounded-xl" />
       </div>
     );
   }