login: embed sulkta-bar for cross-app nav + sign-out

Sign-out now lives in the account.sulkta.com bar (hits Authentik's
default-invalidation-flow). Cauldron already had no per-app sign-out
link in the nav, so this is just the bar embed.

cauldron/templates/_base.html

@@ -681,5 +681,6 @@ button { font-family: inherit; }
 })();
 </script>
 
+{% if session.user %}<script src="https://account.sulkta.com/bar.js" defer></script>{% endif %}
 </body>
 </html>