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

README.md

@@ -35,7 +35,7 @@
 
 - **频道**:唯一的顶层概念,一条只追加的帧流,有自己的 id、密钥、设置和账号成员人数上限。
 - **条目**:频道里的一项内容(文本、JSON、文件),由若干帧组成,可复制、下载、删除。
-- **终端**:加入频道的设备或会话,每个设备有独立终端席位;频道容量按账号成员人数计算,同一账号的多设备只算一人。
+- **成员**:频道里的人是用户(账号);设备只是账号的属性,同一账号的多设备只算一人,请求凭账号会话鉴权。未登录的**访客**以终端密钥 `S` 登记,每个访客终端各算一人。
 - **所有者 / 成员**:所有者持有管理密钥,成员通过频道链接加入。
 - **模式**:`enc`(默认,浏览器端 AES-256-GCM)和 `raw`(负载原样,服务器可见,用于公开或用户自行加密的内容)。