feat: add navigation context processor and update header template

- Created `navigation` context processor to manage `active` context
- Integrated the processor into Django settings
- Simplified `header.html` by removing `{% with %}` for `active` management

templates/_fragments/header.html

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