feat: enhance header context handling and add login page test

- Updated `header.html` to use `with` block for `active` context variable fallback
- Added test to verify login page renders correctly without `active` context

templates/_fragments/header.html

@@ -1,4 +1,5 @@
 <!-- HEADER FRAGMENT -->
+{% with active=active|default:'' %}
 <header>
     <div class="container">
         <div class="header-top">
@@ -72,3 +73,4 @@
         </nav>
     </div>
 </header>
+{% endwith %}