feat(web): show a skeleton while a space loads

Draw the page frame instead of a blank screen, fade it in only after 300ms,
and surface load errors with retry and back-to-home actions.

web/src/app/ChannelSkeleton.tsx

@@ -0,0 +1,31 @@
+import { useEffect } from 'react';
+import { Button } from '@/ui/Button';
+import styles from './ChannelSkeleton.module.css';
+
+const ROWS = [72, 48, 88, 36, 64];
+
+/** 频道加载中:先画出页面框架(Apple HIG「别留白屏」),内容到了再替换。超过约 300ms 才可见,避免快的时候闪一下 */
+export function ChannelSkeleton({ error, onRetry, onHome }: { error?: string; onRetry: () => void; onHome: () => void }) {
+  // 与手机聊天页一致:账号入口不浮在骨架屏上
+  useEffect(() => {
+    document.documentElement.dataset.chatShell = '';
+    return () => { delete document.documentElement.dataset.chatShell; };
+  }, []);
+  return <div className={styles.page} role="status" aria-label={error ? '频道加载失败' : '正在加载频道'} aria-busy={!error}>
+    <header className={styles.bar}><span className={`${styles.block} ${styles.title}`} /></header>
+    <aside className={styles.rail}><span className={`${styles.block} ${styles.tile}`} /></aside>
+    <aside className={styles.nav}>{[56, 40, 48].map((w) => <span key={w} className={styles.block} style={{ width: `${w}%` }} />)}</aside>
+    <main className={styles.chat}>
+      {ROWS.map((w, i) => <div key={i} className={styles.message}>
+        <span className={`${styles.block} ${styles.avatar}`} />
+        <div className={styles.lines}><span className={`${styles.block} ${styles.name}`} /><span className={styles.block} style={{ width: `${w}%` }} /></div>
+      </div>)}
+      <div className={`${styles.block} ${styles.composer}`} />
+    </main>
+    <aside className={styles.files}>{[80, 60, 70, 50].map((w) => <span key={w} className={styles.block} style={{ width: `${w}%` }} />)}</aside>
+    {error ? <div className={styles.error} role="alert">
+      <p>频道没能加载：{error}</p>
+      <div><Button variant="primary" onClick={onRetry}>重试</Button><Button variant="ghost" onClick={onHome}>回到首页</Button></div>
+    </div> : null}
+  </div>;
+}