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/webpack/lib/css/CssMergeStyleSheetsRuntimeModule.js

@@ -0,0 +1,57 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Natsu @xiaoxiaojx
+*/
+
+"use strict";
+
+const RuntimeGlobals = require("../RuntimeGlobals");
+const RuntimeModule = require("../RuntimeModule");
+const Template = require("../Template");
+
+/** @typedef {import("../Chunk")} Chunk */
+
+class CssMergeStyleSheetsRuntimeModule extends RuntimeModule {
+	constructor() {
+		super("css merge stylesheets");
+	}
+
+	/**
+	 * Generates runtime code for this runtime module.
+	 * @returns {string | null} runtime code
+	 */
+	generate() {
+		const { runtimeTemplate } = /** @type {import("../Compilation")} */ (
+			this.compilation
+		);
+
+		return Template.asString([
+			`${RuntimeGlobals.cssMergeStyleSheets} = ${runtimeTemplate.basicFunction(
+				"sheets",
+				[
+					"var sheetsArray = Array.isArray(sheets) ? sheets : [sheets];",
+					"var cssTexts = [];",
+					"for (var i = 0; i < sheetsArray.length; i++) {",
+					Template.indent([
+						"var s = sheetsArray[i];",
+						"if (!s) continue;",
+						"if (typeof s === 'string') {",
+						Template.indent("cssTexts.push(s);"),
+						"} else if (s.cssRules) {",
+						Template.indent([
+							"var rules = s.cssRules;",
+							"for (var j = 0; j < rules.length; j++) {",
+							Template.indent("cssTexts.push(rules[j].cssText);"),
+							"}"
+						]),
+						"}"
+					]),
+					"}",
+					"return cssTexts.join('');"
+				]
+			)};`
+		]);
+	}
+}
+
+module.exports = CssMergeStyleSheetsRuntimeModule;