feat(ui): integrate Notes component into main grid (was dead import)

Per original spec and AUDIT/PLAN item 4. Notes now visible alongside Timer/Music.

frontend/src/App.tsx

@@ -140,6 +140,7 @@ export default function App() {
             <div className="space-y-4">
               <Timer />
               <MusicPlayer />
+              <Notes />
             </div>
 
             {/* Column 3: Chat + Text Input */}