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/index.js

@@ -0,0 +1,64 @@
+const PassThroughDecoder = require('./lib/pass-through-decoder')
+const UTF8Decoder = require('./lib/utf8-decoder')
+
+module.exports = class TextDecoder {
+  constructor(encoding = 'utf8') {
+    this.encoding = normalizeEncoding(encoding)
+
+    switch (this.encoding) {
+      case 'utf8':
+        this.decoder = new UTF8Decoder()
+        break
+      case 'utf16le':
+      case 'base64':
+        throw new Error('Unsupported encoding: ' + this.encoding)
+      default:
+        this.decoder = new PassThroughDecoder(this.encoding)
+    }
+  }
+
+  get remaining() {
+    return this.decoder.remaining
+  }
+
+  push(data) {
+    if (typeof data === 'string') return data
+    return this.decoder.decode(data)
+  }
+
+  // For Node.js compatibility
+  write(data) {
+    return this.push(data)
+  }
+
+  end(data) {
+    let result = ''
+    if (data) result = this.push(data)
+    result += this.decoder.flush()
+    return result
+  }
+}
+
+function normalizeEncoding(encoding) {
+  encoding = encoding.toLowerCase()
+
+  switch (encoding) {
+    case 'utf8':
+    case 'utf-8':
+      return 'utf8'
+    case 'ucs2':
+    case 'ucs-2':
+    case 'utf16le':
+    case 'utf-16le':
+      return 'utf16le'
+    case 'latin1':
+    case 'binary':
+      return 'latin1'
+    case 'base64':
+    case 'ascii':
+    case 'hex':
+      return encoding
+    default:
+      throw new Error('Unknown encoding: ' + encoding)
+  }
+}