fix: add explicit PACKAGE_TYPE::train metadata to training client

Both client-train and client-track now explicitly send package type:
- RTCClient sends "PACKAGE_TYPE::train" before file transfer
- RTCTrackClient sends "PACKAGE_TYPE::track" before file transfer
- Worker explicitly handles both package types

This makes the protocol more explicit and consistent rather than
relying on implicit default behavior.

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: alicup29

sleap_RTC/client/client_class.py

@@ -341,9 +341,12 @@ async def send_client_file(self, file_path: str = None, output_dir: str = ""):
         if not Path(file_path).exists():
             logging.info("File does not exist.")
             return
-        else: 
+        else:
             logging.info(f"Sending {file_path} to worker...")
 
+            # Send package type indicator
+            self.data_channel.send("PACKAGE_TYPE::train")
+
             # Send output directory (where models will be saved).
             output_dir = "models"
             if self.config_info_list:

sleap_RTC/worker/worker_class.py

@@ -756,6 +756,10 @@ async def on_message(message):
                     self.package_type = "track"
                     logging.info("Received track package (inference mode)")
                     return
+                elif "PACKAGE_TYPE::train" in message:
+                    self.package_type = "train"
+                    logging.info("Received train package (training mode)")
+                    return
 
                 if message == "END_OF_FILE":
                     logging.info("End of file transfer received.")