feat: implement user authentication with login/logout functionality

- Add user login screen with German interface
- Add user icon and dropdown menu in header for authenticated users
- Add password change functionality with proper redirects
- Configure authentication URLs and settings
- Ensure all auth functions redirect to main page instead of admin
- Complete openspec change proposal for login feature

pages/templates/registration/password_change_done.html

@@ -0,0 +1,24 @@
+{% extends "base.html" %}
+{% load static %}
+
+{% block title %}Passwort geändert{% endblock %}
+
+{% block content %}
+<div class="row">
+  <div class="col-md-6 col-md-offset-3">
+    <div class="panel panel-default">
+      <div class="panel-heading">
+        <h3 class="panel-title">Passwort erfolgreich geändert</h3>
+      </div>
+      <div class="panel-body">
+        <div class="alert alert-success">
+          <p>Ihr Passwort wurde erfolgreich geändert.</p>
+        </div>
+        <p>
+          <a href="/" class="btn btn-primary">Zurück zur Startseite</a>
+        </p>
+      </div>
+    </div>
+  </div>
+</div>
+{% endblock %}