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/page2_personal.html

@@ -1,12 +1,12 @@
 {% extends "base.html" %}
 
-{% block title %}Bewerbung - Persönliche Daten{% endblock %}
+{% block title %}Bewerbung - Daten{% endblock %}
 
 {% block header %}Bewerbung: {{ job_name }}{% endblock %}
 
 {% block content %}
 <div class="page-intro">
-    <h2>Schritt 1 von 3: Persönliche Daten</h2>
+    <h2>Schritt 1 von 3: Daten</h2>
     <p>Bitte geben Sie Ihre persönlichen Informationen ein. Alle Felder mit * sind Pflichtfelder.</p>
 </div>