fix(ai): 网络失败时显示友好提示，不再直接展示 Failed to fetch

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/components/ConnectionPlaceholder.tsx

@@ -1,5 +1,6 @@
 import { useMemo } from "react";
 import { useDevice } from "@/contexts/DeviceContext";
+import { isLikelyNetworkFetchError } from "@/lib/networkError";
 import { Loader2, Wifi } from "lucide-react";
 import BottomNav from "@/components/BottomNav";
 import localeZh from "@/locales/data-zh.json";
@@ -50,7 +51,7 @@ export default function ConnectionPlaceholder() {
         <>
           <p className="text-[15px] text-white/80 mb-2">{text.failedTitle}</p>
           <p className="text-[13px] text-white/45 text-center max-w-xs">
-            {error ?? text.failedDesc}
+            {error && !isLikelyNetworkFetchError(error) ? error : text.failedDesc}
           </p>
           <p className="text-[12px] text-white/30 mt-3 text-center max-w-xs">{text.failedHint}</p>
         </>