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

app/[locale]/layout.tsx

@@ -101,6 +101,10 @@ export default async function Layout(props: {
       label: safeLocale === 'de' ? 'Referenzen' : 'References',
       url: `/${safeLocale}/referenzen`
     },
+    { 
+      label: safeLocale === 'de' ? 'Karriere' : 'Career', 
+      url: `/${safeLocale}/${safeLocale === 'de' ? 'karriere' : 'career'}` 
+    },
     { 
       label: safeLocale === 'de' ? 'Messen' : 'Events', 
       url: `/${safeLocale}/${await mapFileSlugToTranslated('messen', safeLocale)}`