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/infrastructure/services/path_resolver_service.rs

@@ -173,6 +173,7 @@ impl PathResolverService {
                     category: Arc::from(category_for(&name, &mime)),
                     size_formatted: format_file_size(sz),
                     owner_id: uid,
+                    sort_date: None,
                 }))
             }
         }