fix(chat): the review's gaps — owner hand-over, live group lines, names, scroll, one tab's sound, uploads on disk, a chat button without the pet

- A group whose creator left or was deleted passes to its longest-standing
  member (ensureOwner), so it can still be managed.
- Group changes (added / removed / renamed / left) are sent live as
  chat_message and never count as unread, here or on the server.
- A disabled account is still named in old chats (people carries
  `disabled`) and never offered for a new one; a deleted one reads
  'Deleted user' instead of '?'.
- Load older keeps the message being read in place.
- With several tabs open only the first to see a message plays its sound /
  system notification (agenthub.chat.notified).
- The people list is read again on New chat and on every room change.
- Uploads go to <data dir>/chat/.incoming/ (multer disk storage) and are
  renamed into place — never held in memory; a refused one is removed.
- With the pet hidden, a chat button in the bottom-right corner (ChatDock,
  server mode) opens the same window, unread count and new-message card
  included.

client/src/lib/api.ts

@@ -553,6 +553,8 @@ export type ChatPerson = {
   username: string;
   displayName: string | null;
   avatarAt: string | null;
+  /** A disabled account: named in old chats, never offered for a new one. */
+  disabled?: boolean;
 };
 /** A file of a chat message (`GET /api/chat/rooms/:id/files/:fileId`). */
 export type ChatFile = { id: string; name: string; type: string; size: number };