feat: 目标不在线时进行提示和更新UI

WebRTCController/app/src/main/java/com/ttstd/controller/MainActivity.java

@@ -269,9 +269,27 @@ public class MainActivity extends AppCompatActivity {
             case "ICE_CANDIDATE":
                 handleIceCandidate(message);
                 break;
+            case "TARGET_OFFLINE":
+                handleTargetOffline(message);
+                break;
         }
     }
 
+    private void handleTargetOffline(SignalMessage message) {
+        String offlineId = message.getToDeviceId();
+        String payload = message.getPayload();
+        String text = (payload != null && !payload.isEmpty())
+                ? payload
+                : "目标被控端不在线，请确认设备已开启并连接服务器";
+        String finalText = text;
+        runOnUiThread(() -> {
+            tvStatus.setText("状态: " + finalText);
+            Toast.makeText(MainActivity.this, finalText, Toast.LENGTH_LONG).show();
+            updateUI(false);
+        });
+        Log.w(TAG, "Target device " + offlineId + " is offline");
+    }
+
     private void handleAnswer(SignalMessage message) {
         try {
             JsonObject payload = gson.fromJson(message.getPayload(), JsonObject.class);