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/locales/data-zh.json

@@ -4,7 +4,7 @@
     "ipMissingDesc": "请在地址栏添加设备 IP 参数后再打开本页，例如：",
     "connecting": "正在连接 {ip}…",
     "failedTitle": "连接失败",
-    "failedDesc": "请检查设备 IP 与网络连接。",
+    "failedDesc": "无法连接设备。请确认本页与设备在同一网络，并检查设备 IP 是否正确。",
     "failedHint": "通过地址栏指定设备，例如 ?ip=10.0.0.119"
   },
   "home": {
@@ -611,6 +611,8 @@
     "thinking": "思考中…",
     "stopped": "（已停止）",
     "requestFailed": "请求失败，请稍后重试",
+    "deviceUnreachable": "无法连接设备。请确认本页与设备在同一网络后再试。",
+    "aiUnreachable": "无法连接 Luxsin AI 服务。请检查网络后再试。",
     "ariaSend": "发送",
     "ariaStop": "停止",
     "ariaNewChat": "新建会话",