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>

assets/css/main.css

@@ -198,6 +198,16 @@ input[type="search"]::-webkit-search-cancel-button {
   opacity: 1;
 }
 
+/* ── full-image zoom overlay ── */
+.img-zoom {
+  position: fixed; inset: 0; z-index: 400;
+  background: rgba(8,6,4,.97);
+  display: none; place-items: center;
+  cursor: zoom-out;
+}
+.img-zoom[data-open="true"] { display: grid; }
+.lb-img { cursor: zoom-in; }
+
 /* ── scrollbar ── */
 ::-webkit-scrollbar { width: 10px; height: 10px; }
 ::-webkit-scrollbar-thumb { background: var(--rule); border-radius: 0; }