[codex] Improve preprocessing layout

[Andrianarivelo]

Summary

• Reworks the preprocessing layout so the data browser and central visuals share the main splitter.
• Improves the file queue panel spacing, labels, selection controls, path hint display, and tooltips.
• Adds a helper for applying logical preprocessing splitter sizes consistently.

Validation

• python -m py_compile pyBer/main.py pyBer/gui_preprocessing.py