feat: add the admin layout shell

templates/_partials/_flashes.html.twig

@@ -0,0 +1,13 @@
+{# Flat inline alerts for now. myep renders these as Toastify toasts, but that means adopting
+   toastify-js as a dependency -- deferred until the toast/OOB work (plan step 7). #}
+{% set levels = { error: 'bg-red-600', warning: 'bg-red-600', success: 'bg-emerald-600', info: 'bg-primary' } %}
+
+{% for level, messages in app.flashes(levels|keys) %}
+    <div class="fixed bottom-4 right-4 z-50 space-y-2">
+        {% for message in messages %}
+            <div class="{{ levels[level] }} text-gray-100 rounded shadow-lg px-4 py-3 text-sm">
+                {{ message|trans }}
+            </div>
+        {% endfor %}
+    </div>
+{% endfor %}