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/aiApi.ts

@@ -11,6 +11,7 @@ import {
   dbToVolume,
   volumeToDb,
 } from "./luxsinApi";
+import { AI_NETWORK_ERROR_CODE, isLikelyNetworkFetchError } from "./networkError";
 
 // Allow override via Vite env / localStorage; default to same origin in prod.
 export function getAIBaseUrl(): string {
@@ -120,7 +121,13 @@ export interface QuestionPayload {
 // REST helpers
 // ============================================================
 async function jsonFetch<T>(url: string, init?: RequestInit): Promise<T> {
-  const res = await fetch(url, init);
+  let res: Response;
+  try {
+    res = await fetch(url, init);
+  } catch (error) {
+    if (isLikelyNetworkFetchError(error)) throw new Error(AI_NETWORK_ERROR_CODE);
+    throw error;
+  }
   if (!res.ok) throw new Error(`HTTP ${res.status}: ${await res.text()}`);
   return res.json();
 }
@@ -293,7 +300,7 @@ export function streamQuestion(payload: QuestionPayload, cb: StreamCallbacks): A
       cb.onDone?.();
     } catch (e: any) {
       if (e?.name === "AbortError") return;
-      cb.onError?.(e?.message ?? String(e));
+      cb.onError?.(isLikelyNetworkFetchError(e) ? AI_NETWORK_ERROR_CODE : (e?.message ?? String(e)));
     }
   })();
 
@@ -472,6 +479,12 @@ export async function executeFrontendTool(
         return { ok: false, message: `Unsupported tool: ${block.name}` };
     }
   } catch (e: any) {
+    if (isLikelyNetworkFetchError(e)) {
+      return {
+        ok: false,
+        message: "Device is not reachable. Ask the user to reconnect the device on the same network, then retry.",
+      };
+    }
     return { ok: false, message: e?.message ?? String(e) };
   }
 }