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.

docs/API.md

@@ -3502,7 +3502,7 @@ A **Codex pet** is the `.codex-pet.zip` format Codex Desktop uses (the bundles s
 The pet's window has a rail of chats beside the chat with the pet: direct chats (one per pair of users) and groups. Only a room's MEMBERS read or write it — an admin or analyst included; another user's room answers `404 ENOTFOUND` like a missing one. Local mode answers `400 ELOCALMODE`. Code: `server/lib/chat.js`, `server/routes/chat.js`, client `client/src/components/Sagi/ChatHub.tsx` + `useChat.ts`.
 
 ```
-GET    /api/chat/people                         {people: [{id, username, displayName, avatarAt}]} — every enabled account but you
+GET    /api/chat/people                         {people: [{id, username, displayName, avatarAt, disabled}]} — every account but you; a disabled one is named in old chats, never offered for a new one
 GET    /api/chat/rooms                          {rooms} — yours, newest first: members, lastMessage, lastReadId, clearedId, reads (every member's read mark — "Seen"), unread
 POST   /api/chat/rooms                          {kind: "dm", userId} → the pair's one direct chat (201 new / 200 existing); {kind: "group", name?, userIds} → 201
 PATCH  /api/chat/rooms/:id                      {name?, add?, remove?} — the group's creator only (403 EFORBIDDEN)
@@ -3516,7 +3516,7 @@ POST   /api/chat/rooms/:id/read                 {upTo} — read mark, never move
 GET    /api/chat/rooms/:id/files/:fileId        The file, members only, `sandbox` CSP + `nosniff`; PNG/JPEG/GIF/WebP inline, else attachment
 ```
 
-Files are stored under `<data dir>/chat/<room id>/<32 hex id>` — a name the server makes. Right-click menus in the window: a room (open, mark read, mute notifications — per browser —, copy name, group settings, leave), the rail's pet and **+** (mark all read), a message (copy text, reply, message the sender privately, delete your own), a file (open, download, copy name) and a group member (message privately, remove — the creator). **Deleting a chat** is per person: the messages so far vanish for the caller only (`chat_members.cleared_id`) and a direct chat leaves their list (`hidden`) until a new message arrives; the others keep theirs. What EVERY member has deleted is deleted for real — the messages up to the lowest `cleared_id` and their files on disk, and a direct chat both people deleted, whole (`purge`, also run when a member leaves or is removed). **Seen**: the room shows under your newest message "Seen" (direct chat), "Seen by …" or "Seen by everyone" (group), from `reads`. A group change adds a `system` message (`created`, `renamed`, `left`, `added:<ids>`, `removed:<ids>`). Every change is sent over `/ws` as `chat_message` (`{roomId, message, to}`), `chat_room` (`{roomId, to}`) or `chat_deleted` (`{roomId, messageId, to}`) or `chat_read` (`{roomId, userId, lastReadId, to}`) to exactly the user ids in `to` (`scope.exactAudience`), whatever their role.
+Files are stored under `<data dir>/chat/<room id>/<32 hex id>` — a name the server makes. Right-click menus in the window: a room (open, mark read, mute notifications — per browser —, copy name, group settings, leave), the rail's pet and **+** (mark all read), a message (copy text, reply, message the sender privately, delete your own), a file (open, download, copy name) and a group member (message privately, remove — the creator). **Deleting a chat** is per person: the messages so far vanish for the caller only (`chat_members.cleared_id`) and a direct chat leaves their list (`hidden`) until a new message arrives; the others keep theirs. What EVERY member has deleted is deleted for real — the messages up to the lowest `cleared_id` and their files on disk, and a direct chat both people deleted, whole (`purge`, also run when a member leaves or is removed). A group whose creator left or was deleted passes to its longest-standing member. Group changes (`system` lines) are sent live as `chat_message` and never count as unread. An upload is written to `<data dir>/chat/.incoming/` and moved into place — never held in memory whole. In the browser, only the first tab to see a message plays its sound or shows a system notification; a message from someone in a room not on screen shows as a card on the pet. With the pet hidden, a 💬 button in the bottom-right corner (`ChatDock.tsx`) opens the same chat window. **Seen**: the room shows under your newest message "Seen" (direct chat), "Seen by …" or "Seen by everyone" (group), from `reads`. A group change adds a `system` message (`created`, `renamed`, `left`, `added:<ids>`, `removed:<ids>`). Every change is sent over `/ws` as `chat_message` (`{roomId, message, to}`), `chat_room` (`{roomId, to}`) or `chat_deleted` (`{roomId, messageId, to}`) or `chat_read` (`{roomId, userId, lastReadId, to}`) to exactly the user ids in `to` (`scope.exactAudience`), whatever their role.
 
 ### Session snapshots