feat: add navigation bar with progress indicator and backward navigation

- Created _navigation.html component with 5-step progress indicator
- Added German labels (E-Mail, Persönliche Daten, Motivation, Dokumente, Bestätigung)
- Implemented can_access_page() validation in routes.py
- Users can navigate backward to completed pages (data preserved)
- Sequential forward progression enforced (cannot skip ahead)
- Visual states: completed (green checkmark), current (blue), future (gray)
- Added comprehensive CSS styling with responsive design
- Updated base.html to include navigation component
- Updated page templates to pass current_page context
- Navigation appears on pages 2-5 only
- All 102 tests passing with no regressions

templates/base.html

@@ -12,6 +12,9 @@
             <h1>{% block header %}Bewerbungsformular{% endblock %}</h1>
         </header>
 
+        <!-- Progress Navigation -->
+        {% include '_navigation.html' %}
+
         <main>
             {% with messages = get_flashed_messages(with_categories=true) %}
                 {% if messages %}