feat(modules): add external module registration and defineModule support

- Add `./modules/define` export path pointing to `defineModule.js`
- Implement `registerExternalModules()` to handle modules passed via `zen.config.js`, with env var gating (`ZEN_MODULE_<NAME>=true`)
- Extract `buildAdminConfig()` helper to consolidate admin navigation/page config building
- Refactor `loadModuleConfig()` to use `buildAdminConfig()` and simplify public routes check
- Improve `initializeModuleTables()` to gracefully skip modules without `db.js` instead of erroring
- Update module discovery JSDoc to reflect external module registration support

src/features/provider/ZenProvider.js

@@ -1,12 +1,32 @@
 'use client';
 
+import { useState } from 'react';
 import { ToastProvider, ToastContainer } from '@hykocx/zen/toast';
+import { registerExternalModulePages } from '../../modules/modules.pages.js';
 
-export function ZenProvider({ children }) {
-    return (
-        <ToastProvider>
-            {children}
-            <ToastContainer />
-        </ToastProvider>
-    );
+/**
+ * ZenProvider — root client provider for the ZEN CMS.
+ *
+ * Pass external module configs via the `modules` prop so their
+ * admin pages and public pages are available to the client router.
+ *
+ * @param {Object}   props
+ * @param {Array}    props.modules  - External module configs from zen.config.js
+ * @param {ReactNode} props.children
+ */
+export function ZenProvider({ modules = [], children }) {
+  // Register external module pages once, synchronously, before first render.
+  // useState initializer runs exactly once and does not cause a re-render.
+  useState(() => {
+    if (modules.length > 0) {
+      registerExternalModulePages(modules);
+    }
+  });
+
+  return (
+    <ToastProvider>
+      {children}
+      <ToastContainer />
+    </ToastProvider>
+  );
 }