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 %}

templates/_partials/_nav.html.twig

@@ -0,0 +1,25 @@
+{# Plain nav instead of myep's knp-menu builder: a handful of links does not justify the
+   indirection. Active state is a prefix match on the current route name, which mirrors what
+   myep's menu matcher does with its 'routes' extras. #}
+{% set current_route = app.request.attributes.get('_route') %}
+{% set items = [
+    { route: 'app_dashboard', label: 'Übersicht', icon: 'lucide:layout-grid' },
+    { route: 'app_booking', label: 'Meine Buchungen', icon: 'lucide:calendar-days' },
+] %}
+
+<div class="min-h-full flex grow flex-col gap-y-5 lg:border-r border-gray-200 bg-white md:pr-6">
+    <nav class="flex flex-1 flex-col">
+        <ul role="list" class="space-y-1">
+            {% for item in items %}
+                {% set is_active = current_route starts with item.route %}
+                <li>
+                    <a href="{{ path(item.route) }}"
+                       class="group flex gap-x-3 rounded-md p-2 text-sm leading-6 font-semibold {{ is_active ? 'bg-gray-100 text-gray-900' : 'text-gray-700 hover:bg-gray-50' }}">
+                        {{ ux_icon(item.icon, { class: 'h-5 w-5 shrink-0 ' ~ (is_active ? 'text-gray-600' : 'text-gray-400') }) }}
+                        <span class="flex-1">{{ item.label }}</span>
+                    </a>
+                </li>
+            {% endfor %}
+        </ul>
+    </nav>
+</div>