fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/lines-and-columns/build/index.d.ts

@@ -0,0 +1,13 @@
+export declare type SourceLocation = {
+    line: number;
+    column: number;
+};
+export declare class LinesAndColumns {
+    private string;
+    private offsets;
+    constructor(string: string);
+    locationForIndex(index: number): SourceLocation | null;
+    indexForLocation(location: SourceLocation): number | null;
+    private lengthOfLine;
+}
+export default LinesAndColumns;