Add plugin store feature to backend and frontend

Implemented plugin store API endpoints and types in the backend, including mock data and handlers for listing, detail, and install actions. Added plugin store page, card component, and related logic to the frontend, with navigation and categorized browsing. Updated plugin manager controller and site config to support the new plugin store functionality.

packages/napcat-webui-frontend/src/pages/dashboard/plugin.tsx

@@ -11,13 +11,20 @@ import useDialog from '@/hooks/use-dialog';
 export default function PluginPage () {
   const [plugins, setPlugins] = useState<PluginItem[]>([]);
   const [loading, setLoading] = useState(false);
+  const [pluginManagerNotFound, setPluginManagerNotFound] = useState(false);
   const dialog = useDialog();
 
   const loadPlugins = async () => {
     setLoading(true);
+    setPluginManagerNotFound(false);
     try {
-      const data = await PluginManager.getPluginList();
-      setPlugins(data);
+      const result = await PluginManager.getPluginList();
+      if (result.pluginManagerNotFound) {
+        setPluginManagerNotFound(true);
+        setPlugins([]);
+      } else {
+        setPlugins(result.plugins);
+      }
     } catch (e: any) {
       toast.error(e.message);
     } finally {
@@ -94,7 +101,17 @@ export default function PluginPage () {
           </Button>
         </div>
 
-        {plugins.length === 0 ? (
+        {pluginManagerNotFound ? (
+          <div className="flex flex-col items-center justify-center min-h-[400px] text-center">
+            <div className="text-6xl mb-4">📦</div>
+            <h2 className="text-xl font-semibold text-default-700 dark:text-white/90 mb-2">
+              无插件加载
+            </h2>
+            <p className="text-default-500 dark:text-white/60 max-w-md">
+              插件管理器未加载，请检查 plugins 目录是否存在
+            </p>
+          </div>
+        ) : plugins.length === 0 ? (
           <div className="text-default-400">暂时没有安装插件</div>
         ) : (
           <div className='grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 xl:grid-cols-4 2xl:grid-cols-5 justify-start items-stretch gap-x-2 gap-y-4'>