fix(content): Address all user review feedback

- Replace Mitarbeiter with Mitarbeitenden
- Add specialization sentence to Kabeltiefbau
- Update Werkstatt Kabel link
- Add Karriere to main navigation
- Add Referenzen to Footer
- Fix 404 for EN legal pages by using consistent internal slugs

content/de/messen.mdx

@@ -21,7 +21,7 @@ layout: "fullBleed"
     fairs={[
       { name: 'Intersolar München', date: '23. - 25. Juni 2026', type: 'Messe', location: 'München', url: 'https://www.intersolar.de/' },
       { name: 'Windenergietage Linstow', date: '10. - 12. November 2026', type: 'Fachkongress', location: 'Linstow', url: 'https://www.windenergietage.de/' },
-      { name: 'Werkstatt Kabel 2026', date: '24. - 25. November 2026', type: 'Fachtagung', location: 'Wiesbaden' }
+      { name: 'Werkstatt Kabel 2026', date: '24. - 25. November 2026', type: 'Fachtagung', location: 'Wiesbaden', url: '#' }
     ]}
   />
 </div>