Fix Flow type error in getHighlightedCells loop variables

Explicitly type loop variables as number to avoid incompatible-type
error when Flow infers the union type 0 | 1 | 2 from GridRect fields.

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

Author: LuniMoon

newIDE/app/src/BehaviorsEditor/Editors/AnchorGridMapping.js

@@ -143,8 +143,8 @@ export const getHighlightedCells = (rect: GridRect | null): Set<string> => {
   const cells = new Set<string>();
   if (!rect) return cells;
 
-  for (let col = rect.minCol; col <= rect.maxCol; col++) {
-    for (let row = rect.minRow; row <= rect.maxRow; row++) {
+  for (let col: number = rect.minCol; col <= rect.maxCol; col++) {
+    for (let row: number = rect.minRow; row <= rect.maxRow; row++) {
       cells.add(`${col},${row}`);
     }
   }