fix: set default piano roll tool to 'pen' so brush works

ActiveRollTool default changed from 'select' to 'pen' so clicking
empty space draws notes immediately. Users who switch tool must
switch back to 'pen' to draw.

app/static/js/app.jsx

@@ -4544,7 +4544,7 @@ const AIPresetModal = ({ isOpen, onClose }) => {
 };
 
 const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNotes, onSaveNotes, setSubTabs, onPlayPause, onStop, isPlaying, playPreviewNote, showToast }) => {
-  const [activeRollTool, setActiveRollTool] = React.useState('select');
+  const [activeRollTool, setActiveRollTool] = React.useState('pen');
   const [snapVal, setSnapVal] = React.useState('1/16');
   const [ccMode, setCcMode] = React.useState('velocity');
   const [rollZoom, setRollZoom] = React.useState(60); // local horizontal zoom factor