Add image lightbox — click gallery image to view full-screen

Clicking any image in a post modal opens it full-screen with a dark
overlay. Close by clicking outside, the ✕ button, or pressing Escape.
Added zoom-in cursor and hover opacity to signal clickability.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -184,6 +184,12 @@
     </div>
   </div>
 
+  <!-- LIGHTBOX -->
+  <div id="lightboxOverlay" onclick="closeLightbox()">
+    <button class="lightbox-close" onclick="closeLightbox()">✕</button>
+    <img id="lightboxImg" src="" alt="" onclick="event.stopPropagation()" />
+  </div>
+
   <script src="script.js"></script>
 </body>
 </html>