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/duplexer/test/index.js

@@ -0,0 +1,31 @@
+var through = require("through")
+var test    = require("tape")
+
+var duplex  = require("../index")
+
+var readable = through()
+var writable = through(write)
+var written = 0
+var data = 0
+
+var stream = duplex(writable, readable)
+
+function write() {
+    written++
+}
+
+stream.on("data", ondata)
+
+function ondata() {
+    data++
+}
+
+test("emit and write", function(t) {
+    t.plan(2)
+
+    stream.write()
+    readable.emit("data")
+
+    t.equal(written, 1, "should have written once")
+    t.equal(data, 1, "should have received once")
+})