app: polyfill Array.at

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

package.json

@@ -23,9 +23,9 @@
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "react-markdown": "^8.0.5",
-    "remark-breaks": "^3.0.2",
     "rehype-katex": "^6.0.2",
     "rehype-prism-plus": "^1.5.1",
+    "remark-breaks": "^3.0.2",
     "remark-gfm": "^3.0.1",
     "remark-math": "^5.1.1",
     "sass": "^1.59.2",
@@ -39,6 +39,7 @@
     "@types/react-dom": "^18.0.11",
     "@types/react-katex": "^3.0.0",
     "@types/spark-md5": "^3.0.2",
+    "array.prototype.at": "^1.1.1",
     "cross-env": "^7.0.3",
     "eslint": "^8.36.0",
     "eslint-config-next": "13.2.3",