fix: improve session export and empty chat states

Move session export into the sidebar menu
Add desktop save-and-reveal flow for exported markdown
Show empty-state UI instead of a loading skeleton for empty sessions

packages/ui/src/sync/index.ts

@@ -72,6 +72,7 @@ export {
   useSessionMessageRecords,
   useSessionTextMessages,
   useUserMessageHistory,
+  buildSessionMessageRecordsSnapshot,
 } from "./sync-context"
 
 // Sync operations