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.

static/js/app/state.js

@@ -16,6 +16,7 @@ window.app = {
     isSharedView: false,
     isFavoritesView: false,
     isRecentView: false,
+    isPhotosView: false,
     currentSection: 'files',
     isSearchMode: false,
     shareDialogItem: null,