Prevent text selection during Apple Pencil drawing

Add user-select: none (with WebKit prefix) to the DrawCanvas wrapper so
the browser does not start a text-selection gesture when the pencil drags
across the component.

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

src/pages/DrawCanvas.jsx

@@ -119,7 +119,7 @@ export default function DrawCanvas({ strokes, onStrokesChange, onDone, headerCon
     onStrokesChange([]);
 
   return (
-    <div className="overflow-hidden rounded-3xl border border-slate-200 shadow-sm">
+    <div className="overflow-hidden rounded-3xl border border-slate-200 shadow-sm select-none" style={{ WebkitUserSelect: 'none' }}>
       {/* Toolbar */}
       <div className="flex flex-wrap items-center gap-1 border-b border-slate-200 bg-white px-3 py-2">
         {[