A session's owner can hand out one capability link that opens the display
history without an account: no input, no chat list, no artifacts panel.
The token lives in the URL hash, so it never reaches the server in a request
line or a Referer header.

The link is permanent by decision - no expiry, no revoke. Deleting the chat
removes the share row (FK cascade) and closes it, which is the only switch.

Backend: session_shares table (unique token, FK cascade), POST
/sessions/{id}/share issuing the session's one link (owner or admin; a
read-all permission may read another user's chat but not publish it), and
public GET /shared/{token}[/messages] serving display history only, with
session and owner ids left out of the payload.

Webclient: #share=<token> renders a standalone page with the owner's cards,
rating off, interactive UI components filtered out and scroll-up paging; a
share button in the chat header opens a dialog that creates the link.

Also fixes ChatHeader's rename, which called an undefined `api` and failed
silently.