Add plugin WebUI extension page and API routing support

Introduces a plugin router registry for registering plugin-specific API routes, static resources, and extension pages. Updates the plugin manager and context to expose the router, and implements backend and frontend support for serving and displaying plugin extension pages in the WebUI. Also adds a demo extension page and static resource to the builtin plugin.

packages/napcat-webui-frontend/src/config/site.tsx

@@ -10,6 +10,7 @@ import {
   LuZap,
   LuPackage,
   LuStore,
+  LuPuzzle,
 } from 'react-icons/lu';
 
 export type SiteConfig = typeof siteConfig;
@@ -66,6 +67,11 @@ export const siteConfig = {
       icon: <LuStore className='w-5 h-5' />,
       href: '/plugin_store',
     },
+    {
+      label: '扩展页面',
+      icon: <LuPuzzle className='w-5 h-5' />,
+      href: '/extension',
+    },
     {
       label: '系统终端',
       icon: <LuTerminal className='w-5 h-5' />,