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/core-bridge/account.ts

@@ -6,7 +6,7 @@ const API_BASE = import.meta.env.VITE_WEBDROP_API_URL ?? '/api/v1';
 
 export interface AccountStatus { username: string | null; revision: number; remembered?: boolean; personal?: PersonalProfile; error?: string; unlockUsername?: string }
 export type AccountDevice = AccountDeviceBody;
-export type AccountInvitation = InvitationBody;
+export type AccountInvitation = InvitationBody & { channelName?: string; invitedBy?: string };
 
 export const accountApi = {
   register: (username: string, password: string, remember = true) => callWorker<AccountStatus>('registerAccount', { username, password, remember, apiBase: API_BASE }),