fix: guide users to Auto-Tmux when a persisted session expires (#1336)

* fix: guide users to Auto-Tmux when a persisted session expires

A timed-out terminal session silently reconnected to a fresh shell, so
people running long jobs lost them with no explanation and never learned
about Auto-Tmux. Explain the expiry with a one-click Enable Auto-Tmux
action, let admins default it for new hosts and tune the persistence
timeout from the UI, and move the setting up with copy that says what it
does. The global default stays off.

* style: format terminal expiry notice

src/backend/database/routes/user-settings-routes.ts

@@ -44,6 +44,7 @@ export type HostDefaults = {
   cursorBlink?: boolean;
   enableSessionLogging?: boolean;
   enableCommandHistory?: boolean;
+  autoTmux?: boolean;
 };
 
 async function getAdminActor(