Add full-image zoom overlay to single post pages

A "View full image" button appears in the single-post hero alongside
the return link. Clicking the lightbox main image also opens the same
overlay from any page. The overlay is pure image on a near-black
backdrop; click backdrop, close button, or Escape to dismiss.

Escape is layered: closes zoom first, then lightbox if zoom is not open.

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

layouts/baseof.html

@@ -59,6 +59,23 @@
     {{- $posts | jsonify | safeJS }}
   </script>
 
+  {{/* Pure image zoom — opened by hero button or clicking the lightbox main image */}}
+  <div id="imgZoom" class="img-zoom" role="dialog" aria-modal="true" aria-label="Full size image">
+    <button id="imgZoomClose"
+            class="absolute top-4 right-4 z-10
+                   w-10 h-10 grid place-items-center
+                   border border-white/20 rounded-full
+                   text-white/60 hover:text-white hover:bg-white/10
+                   transition-colors duration-200"
+            aria-label="Close full image">
+      <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor"
+           stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
+        <path d="M18 6 6 18"/><path d="m6 6 12 12"/>
+      </svg>
+    </button>
+    <img id="imgZoomImg" src="" alt="" class="max-w-[96vw] max-h-[96vh] object-contain select-none" />
+  </div>
+
   <script src="/js/app.js" defer></script>
 </body>
 </html>