Fix page meta not updating on client-side post navigation

When a card was clicked, history.pushState changed the URL but left
document.title, description, og:*, and canonical pointing at the
overview page. navigate() also only updated document.title, not meta.

Add setMeta/syncHeadMeta helpers; update title+meta from POSTS data on
card open, and sync all head meta from the fetched document in navigate().

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

static/js/app.js

@@ -20,6 +20,36 @@
     return esc(text).replace(re, '<mark class="hl">$1</mark>');
   }
 
+  function setMeta(attr, key, val) {
+    let el = document.querySelector(`meta[${attr}="${key}"]`);
+    if (val !== null && val !== undefined && val !== '') {
+      if (!el) { el = document.createElement('meta'); el.setAttribute(attr, key); document.head.appendChild(el); }
+      el.setAttribute('content', val);
+    } else if (el) {
+      el.remove();
+    }
+  }
+
+  function syncHeadMeta(doc) {
+    const canon = doc.querySelector('link[rel="canonical"]');
+    const curCanon = document.querySelector('link[rel="canonical"]');
+    if (canon && curCanon) curCanon.href = canon.href;
+    [
+      ['name',     'description'],
+      ['property', 'og:title'],
+      ['property', 'og:description'],
+      ['property', 'og:url'],
+      ['property', 'og:image'],
+      ['property', 'og:image:width'],
+      ['property', 'og:image:height'],
+      ['name',     'twitter:card'],
+      ['name',     'twitter:image'],
+    ].forEach(([attr, key]) => {
+      const src = doc.querySelector(`meta[${attr}="${key}"]`);
+      setMeta(attr, key, src ? src.getAttribute('content') : null);
+    });
+  }
+
   // ── Inverted search index
   const INDEX = (() => {
     const map = new Map();
@@ -377,6 +407,17 @@
     const scopedList = issueId ? POSTS.filter(p => p.issue === issueId) : POSTS;
     lbReferrer = location.href;
     history.pushState({ slug }, '', card.href);
+    if (clicked) {
+      const postTitle = clicked.title + ' — Roux';
+      const postUrl   = new URL(clicked.url, location.origin).href;
+      document.title = postTitle;
+      setMeta('name',     'description',    clicked.description || null);
+      setMeta('property', 'og:title',       postTitle);
+      setMeta('property', 'og:description', clicked.description || null);
+      setMeta('property', 'og:url',         postUrl);
+      const canon = document.querySelector('link[rel="canonical"]');
+      if (canon) canon.href = postUrl;
+    }
     lbOpen(slug, scopedList.length ? scopedList : POSTS);
   });
 
@@ -431,6 +472,7 @@
       }
 
       document.title = doc.title;
+      syncHeadMeta(doc);
       history.pushState({}, '', url);
 
       // Re-init page state