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/locales/de.json

@@ -8,8 +8,14 @@
     "shared": "Geteilt",
     "recent": "Zuletzt verwendet",
     "favorites": "Favoriten",
+    "photos": "Fotos",
     "trash": "Papierkorb"
   },
+  "photos": {
+    "empty_state": "Noch keine Fotos",
+    "empty_hint": "Laden Sie Bilder oder Videos hoch, um sie hier zu sehen",
+    "items_selected": "ausgewählt"
+  },
   "actions": {
     "search": "Dateien suchen...",
     "new_folder": "Neuer Ordner",