Update page title, URL and meta when navigating between lightbox slides

goToSlide() now calls history.replaceState + updates document.title,
description, og:title/description/url and canonical on each slide change.
replaceState is used so swipe/keyboard navigation doesn't pollute history.

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

static/js/app.js

@@ -297,9 +297,22 @@
     if (lbIndex) {
       lbIndex.innerHTML = `<b>${String(lbIdx + 1).padStart(3,'0')}</b> / ${lbList.length}`;
     }
-    lbBuildMeta(lbList[lbIdx]);
+    const p = lbList[lbIdx];
+    lbBuildMeta(p);
     syncThumbs();
     preloadNeighbors(lbIdx);
+    if (p && smooth) {
+      const postTitle = p.title + ' — Roux';
+      const postUrl   = new URL(p.url, location.origin).href;
+      document.title = postTitle;
+      history.replaceState({ slug: p.slug }, '', p.url);
+      setMeta('name',     'description',    p.description || null);
+      setMeta('property', 'og:title',       postTitle);
+      setMeta('property', 'og:description', p.description || null);
+      setMeta('property', 'og:url',         postUrl);
+      const canon = document.querySelector('link[rel="canonical"]');
+      if (canon) canon.href = postUrl;
+    }
   }
 
   function preloadNeighbors(idx) {