Add plugin icon support and caching

Introduce support for plugin icons across backend and frontend. Updates include:

- napcat-onebot: add optional `icon` field to PluginPackageJson.
- Backend (api/Plugin, PluginStore, router): add handlers/utilities to locate and serve plugin icons (`GetPluginIconHandler`, getPluginIconUrl, findPluginIconPath) and wire the route `/api/Plugin/Icon/:pluginId`.
- Cache logic: implement `cachePluginIcon` to fetch GitHub user avatars and store as `data/icon.png` when package.json lacks an icon; invoked after plugin install (regular and SSE flows).
- Frontend: add `icon` to PluginItem, prefer backend-provided icon URL in plugin card (via new getPluginIconUrl util that appends webui_token query param), and add the util to handle token-based image requests.
- Plugin store UI: add a Random category (shuffled), client-side pagination, and reset page on tab/search changes.

These changes let the UI display plugin icons (falling back to author/avatar or Vercel avatars) and cache icons for better UX, while handling auth by passing the token as a query parameter for img src requests.

packages/napcat-webui-backend/src/router/Plugin.ts

@@ -12,7 +12,8 @@ import {
   RegisterPluginManagerHandler,
   PluginConfigSSEHandler,
   PluginConfigChangeHandler,
-  ImportLocalPluginHandler
+  ImportLocalPluginHandler,
+  GetPluginIconHandler
 } from '@/napcat-webui-backend/src/api/Plugin';
 import {
   GetPluginStoreListHandler,
@@ -68,6 +69,7 @@ router.get('/Config/SSE', PluginConfigSSEHandler);
 router.post('/Config/Change', PluginConfigChangeHandler);
 router.post('/RegisterManager', RegisterPluginManagerHandler);
 router.post('/Import', upload.single('plugin'), ImportLocalPluginHandler);
+router.get('/Icon/:pluginId', GetPluginIconHandler);
 
 // 插件商店相关路由
 router.get('/Store/List', GetPluginStoreListHandler);