- Fix probundle exports missing images (double slides/ prefix in storage path)
- Replace manual ZipArchive with PresentationBundle + ProBundleWriter from parser plugin
- Add per-agenda-item download route and buttons for songs and slide items
- Remove text layer from image-only slides in .pro generation
- Fix image conversion: upscale small images, black bars on 2 sides max (contain)
- Add upload warnings for non-16:9 and sub-1920x1080 images (German, non-blocking)
- Update SlideFactory and all tests to use slides/ prefix in stored_filename
- Add 11 new tests (agenda download, image conversion, upload warnings)
