fix(dataset): join data_folder with video path in Qwen3VL iterable dataset (#152)

load_videos() received data_folder but never joined it with the relative
video_path before passing to load_video_qwen_vl_utils(), causing
file:// URLs to use bare relative paths that fail to resolve.

Author: kcz358

src/lmms_engine/datasets/iterable/qwen3_vl_iterable_dataset.py

@@ -59,6 +59,8 @@ def load_videos(self, video_path: str, data_folder=None, fps: int = 1):
         assert (
             self.config.video_backend == "qwen_vl_utils"
         ), "Qwen3VLIterableDataset only supports qwen_vl_utils backend"
+        if data_folder is not None:
+            video_path = os.path.join(data_folder, video_path)
         frames, video_metadata, sample_fps = self.load_video_qwen_vl_utils(video_path, fps)
         return frames, video_metadata, sample_fps