Agentic UI: Fix crash when a toast is replaced in place with a different shape

[shaunandrews]

Related issues

• None (split out of the stu-2162-site-header-actions exploration)

How AI was used in this PR

Claude Code cherry-picked this fix out of a larger exploration branch and verified it (the regression test, lint, typecheck) in isolation.

Proposed Changes

Fixes a crash that occurred when an app toast was updated in place and its content shape changed — for example, a running-sync toast transitioning into its success or failure result, where the result gains or loses a description. Previously React tried to update the existing Notice across that shape change and crashed; now the Notice re-mounts cleanly instead, so the toast transitions smoothly with no crash.

Testing Instructions

1. Run the included regression test: npm test -- apps/ui/src/components/app-toasts/index.test.tsx — it exercises the toast-shape transition directly and must pass.
2. Manual repro (optional): trigger a toast that starts in a running state with a description (e.g. a sync in progress), then let it transition to a result state without a description (e.g. success/failure), and confirm the toast updates cleanly with no crash.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?