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/sync-context.tsx

@@ -1698,7 +1698,7 @@ function getVisibleMessagesForSession(state: State, sessionID: string, previous?
   }
 }
 
-function buildSessionMessageRecordsSnapshot(
+export function buildSessionMessageRecordsSnapshot(
   state: State,
   sessionID: string,
   previous?: SessionMessageRecordsSnapshot,