Add video viewer modal for gallery videos

- Click on video thumbnail to open in large viewer
- Video plays on hover, pauses when mouse leaves
- Close viewer by clicking X or clicking outside video

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/public/index.html

@@ -182,6 +182,12 @@
         <div id="modal-overlay" class="modal-overlay hidden">
             <div id="modal-content" class="modal-content"></div>
         </div>
+
+        <!-- Video Viewer -->
+        <div id="video-viewer" class="video-viewer hidden">
+            <button id="video-viewer-close" class="video-viewer-close">&times;</button>
+            <video id="video-viewer-player" class="video-viewer-player" controls loop></video>
+        </div>
     </div>
 
     <script src="/js/app.js"></script>