feat: Add login/logout button

src/fellchensammlung/templates/fellchensammlung/header.html

@@ -14,6 +14,14 @@
 
     </nav>
     <div class="header-right">
-    <a href="https://codeberg.org/moanos/notfellchen">{% translate "About the software" %}</a>
+        {% if user.is_authenticated %}
+            <div class="profile-card">
+                <p>{{ user.username }}</p>
+                <form action="{% url 'logout' %}" method="post">
+                    {% csrf_token %}
+                    <button type="submit">Ausloggen</button>
+                </form>
+            </div>
+        {% endif %}
     </div>
 </div>