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/check-node-version/tools.js

@@ -0,0 +1,53 @@
+const { execSync } = require("child_process");
+
+module.exports = {
+  node: {
+    getVersion: "node --version",
+    getInstallInstructions(v) {
+      if (hasNvm()) {
+        return `To install node, run \`nvm install ${v}\``;
+      }
+
+      return `To install node, see https://nodejs.org/download/release/v${v}/`;
+    }
+  },
+  npm: {
+    getVersion: "npm --version",
+    getInstallInstructions(v) {
+     return `To install npm, run \`npm install -g npm@${v}\``;
+    }
+  },
+  npx: {
+    getVersion: "npx --version",
+    getInstallInstructions(v) {
+     return `To install npx, run \`npm install -g npx@${v}\``;
+    }
+  },
+  yarn: {
+    getVersion: "yarn --version",
+    getInstallInstructions(v) {
+     return `To install yarn, see https://github.com/yarnpkg/yarn/releases/tag/v${v}`;
+    }
+  },
+  pnpm: {
+    getVersion: "pnpm --version",
+    getInstallInstructions(v) {
+     return `To install pnpm, run \`npm install -g pnpm@${v}\``;
+    }
+  },
+};
+
+function hasNvm() {
+  try {
+   // check for existance of nvm
+   execSync(
+     'nvm',
+     { stdio:[] } // don't care about output
+   );
+  } catch (e) {
+   // no nvm,
+   return false;
+  }
+
+  return true;
+}