feat: serve responsive WebP srcsets for faster mobile browsing

On-demand ?w= resize with a disk cache under webcache/; downloads stay full-resolution.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/templates/base.html

@@ -44,7 +44,8 @@
       if (!target.classList.contains("lightboxable")) return;
       e.preventDefault();
       e.stopPropagation();
-      openLightbox(target.currentSrc || target.src, target.alt);
+      var full = target.getAttribute("data-full-src");
+      openLightbox(full || target.currentSrc || target.src, target.alt);
     });
 
     box.addEventListener("click", function (e) {