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/lib/networkError.ts

@@ -0,0 +1,33 @@
+/** Browser / WebView fetch failures are usually TypeError with an opaque message. */
+
+export const AI_NETWORK_ERROR_CODE = "AI_UNREACHABLE";
+
+const NETWORK_FETCH_HINTS = [
+  "failed to fetch",
+  "networkerror",
+  "network request failed",
+  "load failed",
+  "internet connection appears to be offline",
+];
+
+export function isLikelyNetworkFetchError(error: unknown): boolean {
+  const name = error instanceof Error ? error.name : "";
+  const msg = (error instanceof Error ? error.message : String(error ?? "")).toLowerCase();
+  if (msg === AI_NETWORK_ERROR_CODE.toLowerCase()) return true;
+  if (NETWORK_FETCH_HINTS.some((hint) => msg.includes(hint))) return true;
+  // Some WebViews surface a bare TypeError when fetch cannot complete.
+  return name === "TypeError" && !msg;
+}
+
+export function formatNetworkAwareMessage(
+  template: string,
+  error: unknown,
+  networkCopy: string,
+): string {
+  const raw = error instanceof Error ? error.message : String(error ?? "");
+  const message = isLikelyNetworkFetchError(error) || isLikelyNetworkFetchError(raw) ? networkCopy : raw;
+  if (template.includes("{message}")) {
+    return template.split("{message}").join(String(message));
+  }
+  return isLikelyNetworkFetchError(error) || isLikelyNetworkFetchError(raw) ? networkCopy : template;
+}