feat(web): discord-style chat, members pane, invitations and optimistic send [CI]

- chat: left-aligned discord-style messages with grouped senders, hover
  actions, an auto-growing composer and file cards
- send: render messages and files locally first and replace the
  placeholder when the worker publishes the real item; failures keep the
  text with a retry action
- navigation: space rail plus a flatter channel list with the space name
  header and collapsible groups
- members: right pane tabs (files / members) grouped by online, offline
  and pending users, with a prominent invite action and a top bar shortcut
- notifications: toast host for invite results; invitees get a toast with
  an accept action, and the invitation payload now carries the space name
  and inviter inside the encrypted grant
- users over devices: drop device names, the device list and device
  rename/release; senders show the account (guests show as guest) and the
  share panel only keeps links
- follow contract 7.0.0 (max_members, members_used, members_full, /join)
- docs: ui-design and frontend updated for the new layout and concepts

web/src/features/channel/InviteSheet.tsx

@@ -0,0 +1,44 @@
+import { useState, type FormEvent } from 'react';
+import { errorMessage } from '@/core-bridge/errors';
+import { useToast } from '@/store/toast';
+import { useWorkspace } from '@/store/workspace';
+import { Button } from '@/ui/Button';
+import { Sheet } from '@/ui/Sheet';
+import styles from './MembersPane.module.css';
+
+export function InviteSheet({ onClose, onInvited }: { onClose: () => void; onInvited: () => void }) {
+  const invite = useWorkspace((s) => s.inviteUser);
+  const [username, setUsername] = useState('');
+  const [busy, setBusy] = useState(false);
+  const [error, setError] = useState('');
+
+  async function submit(event: FormEvent) {
+    event.preventDefault();
+    const target = username.trim();
+    if (!target || busy) return;
+    setBusy(true); setError('');
+    try {
+      await invite(target);
+      useToast.getState().push({ kind: 'ok', text: `已向 ${target} 发送邀请` });
+      onInvited(); onClose();
+    } catch (cause) {
+      const message = errorMessage(cause);
+      setError(message);
+      useToast.getState().push({ kind: 'error', text: `邀请失败：${message}` });
+    } finally { setBusy(false); }
+  }
+
+  return <Sheet title="邀请成员" onClose={() => { if (!busy) onClose(); }} placement="top">
+    <form className={styles.invite} onSubmit={(event) => { void submit(event); }}>
+      <label>用户名
+        <input autoFocus maxLength={32} value={username} disabled={busy} placeholder="已注册用户的用户名" onChange={(event) => setUsername(event.target.value)} />
+      </label>
+      <p className={styles.note}>对方登录后会收到通知，接受邀请后加入空间。能否读取加入前的历史，取决于各频道的历史开关。</p>
+      {error ? <p className={styles.error} role="alert">{error}</p> : null}
+      <div className={styles.inviteActions}>
+        <Button variant="ghost" disabled={busy} onClick={onClose}>取消</Button>
+        <Button type="submit" variant="primary" disabled={busy || !username.trim()}>{busy ? '发送中…' : '发送邀请'}</Button>
+      </div>
+    </form>
+  </Sheet>;
+}