- photos: generates a downscaled JPEG thumbnail per image, because serving a
folder of 12-megapixel originals to a grid is unusable on a TV box. Uses
CatmullRom scaling via golang.org/x/image; decodes jpeg/png/gif/webp.
- an undecodable format returns no thumbnail rather than an error, so the grid
falls back to a placeholder.
- images already smaller than the 512px ceiling are passed through untouched.
- library items endpoint now returns library_id, which the viewer needs to
find its siblings for navigation.
- web: PhotoViewer with keyboard and button navigation within the library.
Verified: 900x1200 PNG thumbnailed to a 512-long-edge JPEG and served;
unit tests assert exact dimensions, aspect preservation, and that non-images
produce no thumbnail without erroring.
