UX: Optional logo, format selector, and skip popup redirect

[ptabor]

Summary

Per user feedback:

1. Skips the immediate redirect to an empty tab, relying instead on the on-page link.
2. Removes the hardcoded WDW21 logo default; songbooks now generate cleanly without any graphics if no image is specified in YAML.
3. Adds a dropdown next to the 'Generuj Śpiewnik' button to select between A4 and A5 formats.