feat: render flash messages as toasts

Co-Authored-By: Claude Opus 5 <[email protected]>

templates/layout.html.twig

@@ -31,7 +31,7 @@
 
     {# Swapped out of band after a booking, hence the stable id and the conditional attribute. #}
     {% block flashes %}
-        <div id="flashes" class="fixed bottom-4 right-4 z-50 space-y-2"
+        <div id="flashes"
              {%- if htmx_oob_swap is defined and htmx_oob_swap %} hx-swap-oob="true"{% endif %}>
             {% include '_partials/_flashes.html.twig' %}
         </div>