feat: snapshot restore point UX with undo/restore timeline

Replace per-edit undo buttons with a timeline of restore points:
- Initial PUC (snapshot 0) before first AI edit per session
- Summary card per response with Undo (latest) / Restore to this point
- Clicked restore point shows "Restored" with green highlight
- All buttons disabled during AI streaming
- Extract snapshot logic into AISnapshotStore module

Fix stale $messages reference via _$msgs() live DOM query helper
for deferred operations (click handlers, callbacks) after
SidebarTabs reparents the panel.

Author: abose