- Left/Right navigates carousel slides (instant jump, loops)
- Up/Down navigates between generation batches
- Opening a specific image correctly highlights it in the carousel via initialIndex
- Add galleryModalLightboxNavPrev/Next signals so TextToImage can provide its
own bundle navigation callbacks independently of the gallery's item list
- Fix: gallery nav callbacks were undefined for TextToImage-opened lightboxes
since generated batches are not in the gallery's flatFilteredItems