fix(docs[spa-nav]): always navigate on popstate to fix back-button content update

why: The popstate handler guarded on `e.state && e.state.spa`, but
browsers can deliver null state for the initial history entry even
after replaceState({spa: true}). This caused three symptoms:
- Back button changed URL but didn't update page content
- Sidebar links resolved wrong after back-navigating across depths
- Brand logo href went stale after SPA navigation
what:
- Remove state guard from popstate handler — always call navigate()
- Safe: navigate() falls back to full page load on error, and
  isPop=true prevents pushing duplicate history entries

Author: tony

docs/_static/js/spa-nav.js

@@ -203,8 +203,8 @@
 
   history.replaceState({ spa: true }, "");
 
-  window.addEventListener("popstate", function (e) {
-    if (e.state && e.state.spa) navigate(location.href, true);
+  window.addEventListener("popstate", function () {
+    navigate(location.href, true);
   });
 
   // --- Hover prefetch ---