feat(photos): add Photos timeline view with lightbox and infinite scroll

Backend: new GET /api/photos endpoint with cursor-based pagination that
queries image/video files sorted by EXIF captured_at (falling back to
created_at), joining file_metadata for sort dates.

Frontend: dense photo grid grouped by day with lazy-loaded thumbnails,
IntersectionObserver infinite scroll, multi-select with batch
download/delete, and a full-screen lightbox with prev/next navigation,
EXIF metadata display, and download/favorite/delete toolbar.

Includes navigation wiring, CSS (with dark theme), and i18n translations
for all 9 locales.

src/interfaces/api/handlers/mod.rs

@@ -11,6 +11,7 @@ pub mod favorites_handler;
 pub mod file_handler;
 pub mod folder_handler;
 pub mod i18n_handler;
+pub mod photos_handler;
 pub mod recent_handler;
 pub mod search_handler;
 pub mod share_handler;