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/login.html

@@ -0,0 +1,43 @@
+{% extends "base.html" %}
+{% load static %}
+
+{% block title %}Anmelden{% endblock %}
+
+{% block content %}
+<div class="row">
+  <div class="col-md-4 col-md-offset-4">
+    <div class="panel panel-default">
+      <div class="panel-heading">
+        <h3 class="panel-title">Anmelden</h3>
+      </div>
+      <div class="panel-body">
+        <form method="post">
+          {% csrf_token %}
+          
+          {% if form.errors %}
+            <div class="alert alert-danger">
+              <p>Ihr Benutzername und Passwort stimmen nicht überein. Bitte versuchen Sie es erneut.</p>
+            </div>
+          {% endif %}
+          
+          <div class="form-group">
+            <label for="id_username">Benutzername:</label>
+            <input type="text" name="username" class="form-control" id="id_username" required autofocus>
+          </div>
+          
+          <div class="form-group">
+            <label for="id_password">Passwort:</label>
+            <input type="password" name="password" class="form-control" id="id_password" required>
+          </div>
+          
+          <div class="form-group">
+            <button type="submit" class="btn btn-primary">Anmelden</button>
+          </div>
+          
+          <input type="hidden" name="next" value="{{ next }}">
+        </form>
+      </div>
+    </div>
+  </div>
+</div>
+{% endblock %}