Fix Default Dataset Not Loading on Startup and Improve Prediction Display After Annotation Selection

[caoTayTang]

1. Fix: Default Dataset Not Loading on Startup
When the GUI first opens, and user click load dataset will throw an error:

AttributeError: 'MainWindow' object has no attribute 'dataset_split'

This is now resolved by ensuring dataset_split is properly initialized before calling load_dataset.

**2. Improvement: Display Predictions Immediately After Annotation

Previously, after selecting an annotation, the predicted boxes were not displayed until additional user interaction ().
This update ensures that predictions (bounding boxes, etc.) are shown immediately upon annotation selection (increment_index, decrement_index, goto_sample_index, on_frame_selected, ...)

These changes improve the initial UX and make annotation review more seamless.