Add a privacy policy page and update footer links

Adds the 'Datenschutz' (privacy policy) page with relevant information and integrates it into the application's routing and footer navigation.

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client/src/components/footer.tsx

@@ -78,6 +78,11 @@ export default function Footer() {
                   <span className="text-muted-foreground cursor-pointer hover:text-primary transition-colors" data-testid="link-footer-impressum">Impressum</span>
                 </Link>
               </li>
+              <li>
+                <Link href="/datenschutz">
+                  <span className="text-muted-foreground cursor-pointer hover:text-primary transition-colors" data-testid="link-footer-datenschutz">Datenschutz</span>
+                </Link>
+              </li>
             </ul>
           </div>
           <div>