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/fast-diff/diff.d.ts

@@ -0,0 +1,21 @@
+declare function diff(
+  text1: string,
+  text2: string,
+  cursorPos?: number | diff.CursorInfo,
+  cleanup?: boolean
+): diff.Diff[];
+
+declare namespace diff {
+  type Diff = [-1 | 0 | 1, string];
+
+  const DELETE: -1;
+  const INSERT: 1;
+  const EQUAL: 0;
+
+  interface CursorInfo {
+    oldRange: { index: number; length: number };
+    newRange: { index: number; length: number };
+  }
+}
+
+export = diff;