[codex] support media srcset for lightbox

[haltakov]

Summary

Adds a direct optional srcset field to media entries in the gallery schema and passes it through resolved theme data as image.srcset. Themes then emit that value as data-pswp-srcset, allowing PhotoSwipe to choose an appropriately sized full-size image instead of always loading the original.

This keeps responsive lightbox image data as a sibling of thumbnail on each media item, rather than introducing a separate nested lightbox schema.

Companion app PR: https://github.com/SimplePhotoGallery/app/pull/189

Impact

• Gallery JSON can now include media-level srcset strings.
• Core theme rendering supports responsive PhotoSwipe image candidates.
• New custom themes generated from the base template include the same attribute.
• Configuration docs now document the media-level srcset field.

Validation

• yarn workspace @simple-photo-gallery/common compile
• yarn workspace simple-photo-gallery compile
• yarn workspace simple-photo-gallery test lightbox-srcset.test.ts --runInBand
• yarn workspace simple-photo-gallery test smoke.test.ts --runInBand