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/index.html

@@ -14,6 +14,8 @@
     <link rel="stylesheet" href="/css/views/recent.css">
     <link rel="stylesheet" href="/css/views/shared.css">
     <link rel="stylesheet" href="/css/views/trash.css">
+    <link rel="stylesheet" href="/css/views/photos.css">
+    <link rel="stylesheet" href="/css/views/photosLightbox.css">
 
     <!-- Scripts (defer: download in parallel, execute in order, after HTML parsed) -->
     <script defer src="/js/core/i18n.js"></script>
@@ -32,6 +34,8 @@
     <script defer src="/js/features/files/search.js"></script>
     <script defer src="/js/features/library/favorites.js"></script>
     <script defer src="/js/features/library/recent.js"></script>
+    <script defer src="/js/features/library/photos.js"></script>
+    <script defer src="/js/features/library/photosLightbox.js"></script>
     <script defer src="/js/features/sharing/fileSharing.js"></script>
     <script defer src="/js/views/shared/sharedView.js"></script>
     <script defer src="/js/features/files/inlineViewer.js"></script>
@@ -81,6 +85,10 @@
                 <i class="fas fa-star"></i>
                 <span data-i18n="nav.favorites">Favorites</span>
             </div>
+            <div class="nav-item" id="nav-photos">
+                <i class="fas fa-images"></i>
+                <span data-i18n="nav.photos">Photos</span>
+            </div>
             <div class="nav-item">
                 <i class="fas fa-trash"></i>
                 <span data-i18n="nav.trash">Trash</span>