Deleted session tab hangs app and leaks RAM when clicked

[Myestery]

Bug

When the active session is deleted, the session view stays clickable (sidebar entry persists during the post-delete refetch window, and the URL still resolves to the deleted id). Clicking it remounts SessionPage against a missing session, the 3s title poll swallows the 404 and keeps running, and the workspace event sync stays subscribed. Result: UI hangs and RAM grows unbounded.

Repro

1. Open a session (it becomes active in the URL /session/{id}).
2. Delete it from the sidebar (or any path).
3. Either click the still-visible sidebar entry, or stay on the now-stale URL.
4. App hangs; memory climbs in Activity Monitor / devtools heap.

Fix

PR #1597 — optimistic sidebar removal + navigate-before-delete when active, plus 404-aware cleanup in the title poll.

hanging.mp4