fix: add capture flag to Ctrl+S keydown handler to prevent browser save

app/static/js/app.jsx

@@ -7428,8 +7428,8 @@ const App = () => {
         return;
       }
     };
-    window.addEventListener('keydown', handler);
+    window.addEventListener('keydown', handler, { capture: true });
-    return () => window.removeEventListener('keydown', handler);
+    return () => window.removeEventListener('keydown', handler, { capture: true });
   }, []);
 
   // ── Temp Tab: draw isolated waveform ──