feat: add curriculum vitae page with secure download

Add dedicated CV page for HR professionals and recruiters with secure Tresorit download link.

Changes:
- Add CV content page at /cv/ with German text
- Configure CV navigation menu item "Curriculum Vitae"
- Enhance navigation template with active state handling for menu items
- Add secure external link with target="_blank" and security attributes (noopener, noreferrer, nofollow)
- Include email verification notice and information about certificates/references availability
- Create OpenSpec proposal for content-management capability

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

layouts/partials/nav.html

@@ -5,7 +5,7 @@
     </li>
     {{ range .Site.Menus.main }}
     <li class="nav-item">
-      <a class="nav-link" href="{{ .URL }}">{{ .Name }}</a>
+      <a class="nav-link{{ if $.IsMenuCurrent "main" . }} active{{ end }}" href="{{ .URL }}">{{ .Name }}</a>
     </li>
     {{ end }}
   </ul>