fix(web): run exit handling when attaching to exited terminals

Mount synchronization now starts from the empty "closed" state like the
previous xterm mount did, so a viewport attaching to a session that
already exited still writes the system message and fires
onSessionExited once.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
6e17ad969aWout Stiens committed on 7/31/2026, 6:48:21 AMparent73a5323
1 file changedLine totals unavailable