app: polyfill Array.at

This fixes compatibility issue with older browsers like WeChat webview.
The summary feature now works as expected.

app/store/app.ts

@@ -11,6 +11,10 @@ import { trimTopic } from "../utils";
 
 import Locale from "../locales";
 
+if (!Array.prototype.at) {
+  require('array.prototype.at/auto');
+}
+
 export type Message = ChatCompletionResponseMessage & {
   date: string;
   streaming?: boolean;