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/Notepad.tsx

@@ -9,7 +9,6 @@ const SAVE_DELAY_MS = 400;
 export function Notepad({ defaultOpen = false }: { defaultOpen?: boolean }) {
   const snapshot = useWorkspace((s) => s.snapshot);
   const setSnapshot = useWorkspace((s) => s.setSnapshot);
-  const editor = useWorkspace((s) => s.devices.find((d) => d.editing && !d.self)?.name);
   const [open, setOpen] = useState(defaultOpen);
   const [draft, setDraft] = useState<string | null>(null);
   const timer = useRef<number | undefined>(undefined);
@@ -31,7 +30,6 @@ export function Notepad({ defaultOpen = false }: { defaultOpen?: boolean }) {
         <span className={`${styles.chev} ${open ? styles.open : ''}`}><Icon name="chevronRight" size={16} /></span>
         <span className={styles.title}>便签</span>
         {open ? null : <span className={styles.preview}>{firstLine}</span>}
-        {editor ? <span className={styles.editing}>{editor} 正在编辑</span> : null}
       </button>
       {open ? (
         <textarea