feat: apply check via header checkbox after drag-selecting rows

After dragging to select rows, checkboxes appear in the Check Ying
and Check Anna column headers showing the count of selected rows.
Clicking one marks or unmarks that column for all selected rows,
then clears the selection. Footer shows a hint with a cancel button.

Co-Authored-By: claude-flow <ruv@ruv.net>

src/components/PricingView.tsx

@@ -2250,11 +2250,11 @@ export function PricingView({ data, headers, onSaveRow, onCaptureState, onEdit,
           </p>
           {dragSelectedRows.size > 0 && (
             <span className="flex items-center gap-1.5 text-xs text-blue-400 bg-blue-500/10 border border-blue-500/20 px-2 py-0.5 rounded mb-1">
-              {dragSelectedRows.size} selected
+              {dragSelectedRows.size} filas — marca Check Ying o Anna en la cabecera
               <button
                 onClick={() => setDragSelectedRows(new Set())}
                 className="ml-1 text-blue-300 hover:text-white transition-colors"
-                title="Clear selection"
+                title="Cancelar selección"
               >
                 <X className="w-3 h-3" />
               </button>