fix(lightbox): slideshow navigation writes browser history

goToSlide() was replaceState()-ing, so paging through the viewer left a
single history entry — browser Back skipped every plate you'd looked at.
It now pushState()s on each user-driven slide (guarded to fire only when the
index actually changes), popstate slides the viewer back/forward without
stacking entries, and the first entry on a directly-loaded plate page is
tagged with its slug so Back to it is handled in-place.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZPmxGywFnAhmYJB1eh9fm

static/js/app.js

@@ -289,9 +289,14 @@
     preloadNeighbors(lbIdx);
   }
 
-  function goToSlide(idx, smooth = true) {
+  // smooth  — animate the track (false = jump, used when first opening)
+  // record  — push a history entry + sync <title>/meta (default follows `smooth`;
+  //           popstate passes false so back/forward doesn't stack entries)
+  function goToSlide(idx, smooth = true, record = smooth) {
     if (!lbTrack) return;
-    lbIdx = Math.max(0, Math.min(idx, lbList.length - 1));
+    const next = Math.max(0, Math.min(idx, lbList.length - 1));
+    const changed = next !== lbIdx;
+    lbIdx = next;
     lbTrack.style.transition = smooth ? '' : 'none';
     lbTrack.style.transform  = `translateX(-${lbIdx * 100}%)`;
     if (!smooth) lbTrack.getBoundingClientRect(); // force reflow
@@ -302,17 +307,17 @@
     lbBuildMeta(p);
     syncThumbs();
     preloadNeighbors(lbIdx);
-    if (p && smooth) {
+    if (p) {
       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;
+      if (record && changed) history.pushState({ slug: p.slug }, '', p.url);
     }
   }
 
@@ -440,15 +445,19 @@
   // Handle browser back/forward.
   window.addEventListener('popstate', e => {
     const slug = e.state && e.state.slug;
-    // Entry points at a plate we already know about → show it in the lightbox now,
-    // no page fetch needed (covers sliding + card-opened plates).
     if (slug && POSTS.some(p => p.slug === slug)) {
+      // Already in the viewer with this plate in the list → just slide to it.
+      if (lb && lb.dataset.open === 'true') {
+        const i = lbList.findIndex(p => p.slug === slug);
+        if (i !== -1) { goToSlide(i, true, false); return; }
+      }
+      // Otherwise (re)open the viewer at this plate — no page fetch needed.
       const opened = POSTS.find(p => p.slug === slug);
       const scoped = POSTS.filter(p => p.issue === opened.issue);
       lbOpen(slug, scoped.length ? scoped : POSTS);
       return;
     }
-    // Otherwise re-render #content for the URL we landed on (also closes the lightbox).
+    // Not a plate → re-render #content for the URL we landed on (also closes the viewer).
     navigate(location.href, { push: false });
   });
 
@@ -562,6 +571,8 @@
     const opened = POSTS.find(p => p.slug === window.__ROUX_OPEN_SLUG);
     const issueId = opened ? opened.issue : null;
     const scoped = issueId ? POSTS.filter(p => p.issue === issueId) : POSTS;
+    // Tag this first history entry so back-navigation to it is handled by the viewer.
+    if (opened) history.replaceState({ slug: opened.slug }, '', location.href);
     lbOpen(window.__ROUX_OPEN_SLUG, scoped.length ? scoped : POSTS);
   }