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-en.json

@@ -4,7 +4,7 @@
     "ipMissingDesc": "Add the device IP in the address bar before opening this page, for example:",
     "connecting": "Connecting to {ip}…",
     "failedTitle": "Connection failed",
-    "failedDesc": "Check the device IP and your network connection.",
+    "failedDesc": "Can't reach the device. Make sure this page and the device are on the same network, and that the IP is correct.",
     "failedHint": "Specify the device in the URL, e.g. ?ip=10.0.0.119"
   },
   "home": {
@@ -913,6 +913,8 @@
     "thinking": "Thinking…",
     "stopped": "(Stopped)",
     "requestFailed": "Request failed. Please try again later.",
+    "deviceUnreachable": "Can't reach the device. Check that this page and the device are on the same network, then try again.",
+    "aiUnreachable": "Can't reach the Luxsin AI service. Check your internet connection and try again.",
     "ariaSend": "Send",
     "ariaStop": "Stop",
     "ariaNewChat": "New chat",