Add image download proxy support to OneBot

Introduces an 'imageDownloadProxy' config option to OneBot, allowing image downloads via a specified HTTP proxy. Updates the file download logic in napcat-common to use the undici library for proxy support, and propagates the new config through backend, frontend, and type definitions. Also adds undici as a dependency.

packages/napcat-onebot/config/config.ts

@@ -82,6 +82,7 @@ export const OneBotConfigSchema = Type.Object({
   musicSignUrl: Type.String({ default: '' }),
   enableLocalFile2Url: Type.Boolean({ default: false }),
   parseMultMsg: Type.Boolean({ default: false }),
+  imageDownloadProxy: Type.String({ default: '' }),
 });
 
 export type OneBotConfig = Static<typeof OneBotConfigSchema>;