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/text-decoder/lib/pass-through-decoder.js

@@ -0,0 +1,19 @@
+const b4a = require('b4a')
+
+module.exports = class PassThroughDecoder {
+  constructor(encoding) {
+    this.encoding = encoding
+  }
+
+  get remaining() {
+    return 0
+  }
+
+  decode(data) {
+    return b4a.toString(data, this.encoding)
+  }
+
+  flush() {
+    return ''
+  }
+}