Fix Apple Pencil scrolling during draw by setting touchAction none

touchAction: 'pan-y pinch-zoom' let the browser intercept vertical
pencil movement as a scroll gesture before JavaScript could prevent it.
Setting 'none' hands all pointer input to our handlers exclusively.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/DrawCanvas.jsx

@@ -232,7 +232,7 @@ export default function DrawCanvas({ strokes, onStrokesChange, onDone, headerCon
           style={{
             zIndex: 2,
             cursor: drawTool === 'eraser' ? 'cell' : 'crosshair',
-            touchAction: 'pan-y pinch-zoom',
+            touchAction: 'none',
           }}
           onPointerDown={onPointerDown}
           onPointerMove={onPointerMove}