In-post event gallery block + lightbox; mobile polish

- Reusable "Photos from the event" grid (shows 5 thumbnails + "+N more") that
  opens a full-screen lightbox (arrows / keyboard / swipe / Esc); added to the
  Meet Student Slovenia 2024 post as the pattern
- New css/components/_lightbox.css + setupLightbox() in main.js (injects one
  shared overlay, wires any .js-lightbox grid)
- Mobile polish: gallery albums go single-column on phones, two on tablets;
  event-gallery grid 3-up on mobile; lightbox sizing/controls tuned for small
  screens

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

css/main.css

@@ -26,6 +26,7 @@
 @import 'components/_buttons.css';
 @import 'components/_cards.css';
 @import 'components/_lang-banner.css';
+@import 'components/_lightbox.css';
 
 /* 4. Page-specific Styles */
 @import 'pages/_home.css';