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

VorgabenUI/urls.py

@@ -18,6 +18,7 @@ from django.contrib import admin
 from django.urls import include, path, re_path
 from django.conf import settings
 from django.conf.urls.static import static
+from django.contrib.auth import views as auth_views
 import dokumente.views
 import pages.views
 import referenzen.views
@@ -32,6 +33,11 @@ urlpatterns = [
     path('stichworte/', include("stichworte.urls")),
     path('referenzen/', referenzen.views.tree, name="referenz_tree"),
     path('referenzen/<str:refid>/', referenzen.views.detail, name="referenz_detail"),
+    # Authentication URLs
+    path('login/', auth_views.LoginView.as_view(template_name='registration/login.html'), name='login'),
+    path('logout/', auth_views.LogoutView.as_view(next_page='/'), name='logout'),
+    path('password_change/', auth_views.PasswordChangeView.as_view(template_name='registration/password_change.html', success_url='/'), name='password_change'),
+    path('password_change/done/', auth_views.PasswordChangeDoneView.as_view(template_name='registration/password_change_done.html'), name='password_change_done'),
 ]
 
 # Serve static files