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/path-is-inside/lib/path-is-inside.js

@@ -0,0 +1,28 @@
+"use strict";
+
+var path = require("path");
+
+module.exports = function (thePath, potentialParent) {
+    // For inside-directory checking, we want to allow trailing slashes, so normalize.
+    thePath = stripTrailingSep(thePath);
+    potentialParent = stripTrailingSep(potentialParent);
+
+    // Node treats only Windows as case-insensitive in its path module; we follow those conventions.
+    if (process.platform === "win32") {
+        thePath = thePath.toLowerCase();
+        potentialParent = potentialParent.toLowerCase();
+    }
+
+    return thePath.lastIndexOf(potentialParent, 0) === 0 &&
+		(
+			thePath[potentialParent.length] === path.sep ||
+			thePath[potentialParent.length] === undefined
+		);
+};
+
+function stripTrailingSep(thePath) {
+    if (thePath[thePath.length - 1] === path.sep) {
+        return thePath.slice(0, -1);
+    }
+    return thePath;
+}