Add Gallery page (per-event albums + lightbox); remove video-thumbnail blur

- New multilingual Gallery page (en/mk/si) with 9 event albums and a
  keyboard-accessible lightbox (prev/next scoped per album)
- Reuses existing event photos; links each album to its event post
- New css/pages/_gallery.css + setupGallery() lightbox in main.js
- "Gallery" link added to the "What We Do" nav (desktop + mobile) site-wide
- Gallery URLs added to sitemap.xml with hreflang
- Landing "Every Student Has a Story": removed backdrop blur on the video
  thumbnail, kept the bluish gradient tint

Note: images/original/ (458 MB of raw source photos, gitignored + unused by
the site) moved out of the working tree to reclaim disk.

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

css/pages/_home.css

@@ -178,7 +178,6 @@
     width: 100%;
     height: 100%;
     background: linear-gradient(127.76deg, rgba(19, 49, 60, 0.92) -13.16%, rgba(39, 77, 90, 0.08) 98.5%);
-    backdrop-filter: blur(6px);
     border-radius: 16px;
 }