Fix logout 405: use POST form instead of GET link

Django 5+ LogoutView rejects GET requests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backlogger/templates/backlogger/list.html

@@ -157,7 +157,7 @@
 <header class="site-header">
   <a class="brand" href="/">k-boris.tech</a>
   <nav>
-    <a href="{% url 'logout' %}">Log out</a>
+    <form method="post" action="{% url 'logout' %}" style="display:inline">{% csrf_token %}<button type="submit" style="background:none;border:none;color:#64748b;font-size:0.85rem;cursor:pointer;padding:0">Log out</button></form>
   </nav>
 </header>