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/@webassemblyjs/utf8/test/index.js

@@ -0,0 +1,13 @@
+const { assert } = require("chai");
+
+const { decode, encode } = require("../lib");
+
+describe("UTF8", () => {
+  it("should f-1(f(x)) = x", () => {
+    assert.equal(decode(encode("foo")), "foo");
+    assert.equal(decode(encode("éé")), "éé");
+
+    // TODO(sven): utf8 encoder fails here
+    // assert.equal(decode(encode("🤣见見")), "🤣见見");
+  });
+});