Update queryClient calls to use array keys

The useQuery hook uses an array format for queryKey (['notes']), but the
mutation's getQueryData and setQueryData calls were using a string format
('notes'). In TanStack Query v4+, these are treated as completely different
cache keys, preventing the optimistic cache update from working.

This caused the UI to not update after adding a new note because the mutation
was updating a different cache entry than the one being watched by useQuery.

Changed queryClient.getQueryData('notes') to queryClient.getQueryData(['notes'])
and queryClient.setQueryData('notes', ...) to queryClient.setQueryData(['notes'], ...)
to ensure cache consistency and proper UI updates.

Author: ebubecodes

src/content/6/fi/osa6d.md

@@ -380,8 +380,8 @@ const App = () => {
     mutationFn: createNote,
     // highlight-start
     onSuccess: (newNote) => {
-      const notes = queryClient.getQueryData('notes')
-      queryClient.setQueryData('notes', notes.concat(newNote))
+      const notes = queryClient.getQueryData(['notes'])
+      queryClient.setQueryData(['notes'], notes.concat(newNote))
     // highlight-end
     }
   })