feat(showcase): click-to-view full-size image/video in recipe and model modals (#926)

- Add MediaViewer overlay for full-size image/video display with prev/next
  navigation, direction keys, counter, and adjacent preloading
- Recipe modal: click preview image/video opens full-size viewer
- Model showcase: click any example image/video opens viewer with full
  gallery navigation; blurred NSFW content opens directly to clear view
- Use Map<Element, number> for DOM-index mapping instead of URL comparison
  to avoid index mismatch from lazy-loaded vs data-attribute URLs

static/css/style.css

@@ -39,6 +39,7 @@
 @import 'components/keyboard-nav.css'; /* Add keyboard navigation component */
 @import 'components/statistics.css'; /* Add statistics component */
 @import 'components/sidebar.css'; /* Add sidebar component */
+@import 'components/media-viewer.css';
 
 .initialization-notice {
     display: flex;