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-frontend/src/controllers/plugin_manager.ts

@@ -26,6 +26,8 @@ export interface PluginItem {
   homepage?: string;
   /** 仓库链接 */
   repository?: string;
+  /** 插件图标 URL（由后端返回） */
+  icon?: string;
 }
 
 /** 扩展页面信息 */