Quite some work done here.

templates/base.html.twig

@@ -14,10 +14,23 @@
     <nav>
         {% set pathinfo = app.request.pathinfo %}
         <a href="/">{{ 'nav.home'|trans }}</a> |
-        <a href="/game">{{ 'nav.game'|trans }}</a>
+        <a href="/game">{{ 'nav.game'|trans }}</a> |
+        {% if app.user %}
+            <a href="{{ path('app_logout') }}">Logout</a>
+        {% else %}
+            <a href="{{ path('app_register') }}">Register</a> |
+            <a href="{{ path('app_login') }}">Login</a>
+        {% endif %}
     </nav>
 </header>
 <main>
+    {% for label, messages in app.flashes %}
+        {% for message in messages %}
+            <div class="alert alert-{{ label }}">
+                {{ message }}
+            </div>
+        {% endfor %}
+    {% endfor %}
     {% block body %}{% endblock %}
 </main>
 <footer>