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/markdown-it/lib/presets/zero.js

@@ -0,0 +1,62 @@
+// "Zero" preset, with nothing enabled. Useful for manual configuring of simple
+// modes. For example, to parse bold/italic only.
+
+'use strict';
+
+
+module.exports = {
+  options: {
+    html:         false,        // Enable HTML tags in source
+    xhtmlOut:     false,        // Use '/' to close single tags (<br />)
+    breaks:       false,        // Convert '\n' in paragraphs into <br>
+    langPrefix:   'language-',  // CSS language prefix for fenced blocks
+    linkify:      false,        // autoconvert URL-like texts to links
+
+    // Enable some language-neutral replacements + quotes beautification
+    typographer:  false,
+
+    // Double + single quotes replacement pairs, when typographer enabled,
+    // and smartquotes on. Could be either a String or an Array.
+    //
+    // For example, you can use '«»„“' for Russian, '„“‚‘' for German,
+    // and ['«\xA0', '\xA0»', '‹\xA0', '\xA0›'] for French (including nbsp).
+    quotes: '\u201c\u201d\u2018\u2019', /* “”‘’ */
+
+    // Highlighter function. Should return escaped HTML,
+    // or '' if the source string is not changed and should be escaped externaly.
+    // If result starts with <pre... internal wrapper is skipped.
+    //
+    // function (/*str, lang*/) { return ''; }
+    //
+    highlight: null,
+
+    maxNesting:   20            // Internal protection, recursion limit
+  },
+
+  components: {
+
+    core: {
+      rules: [
+        'normalize',
+        'block',
+        'inline'
+      ]
+    },
+
+    block: {
+      rules: [
+        'paragraph'
+      ]
+    },
+
+    inline: {
+      rules: [
+        'text'
+      ],
+      rules2: [
+        'balance_pairs',
+        'text_collapse'
+      ]
+    }
+  }
+};