fix: harden html responses and cache chat sidebar data

Use strict template response handling and sanitized template user context, then add an in-process conversation archive cache with mutation-driven invalidation for chat sidebar renders.

html-router/templates/auth/_layout.html

@@ -7,4 +7,5 @@
   {% block auth_content %}
   {% endblock %}
 </div>
+<div id="toast-container" class="fixed bottom-4 right-4 z-50 space-y-2"></div>
 {% endblock %}