Paginate the gallery grid with 60 photos per page

Emit /page/N/ pages alongside index.html and render a pager at the
bottom of each page. Ships as a plain link that navigates without JS,
and an IntersectionObserver enhancement that fetches the next page and
appends tiles for an infinite-scroll feel.

static/style.css

@@ -163,6 +163,30 @@ body {
   transform: scale(1.05);
 }
 
+/* ── Pager ─────────────────────────────────────────────────── */
+
+.pager {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 24px 20px 8px;
+  font-family: monospace;
+  font-size: 13px;
+  letter-spacing: 0.04em;
+}
+
+.pager a {
+  color: var(--aqua);
+  text-decoration: none;
+}
+
+.pager a:hover {
+  color: var(--fg2);
+}
+
+.pager-newer { margin-right: auto; }
+.pager-older { margin-left: auto; }
+
 /* ── Single photo page ─────────────────────────────────────── */
 
 .photo-page {