fix(ui): improve header navigation contrast on dark theme and add frontend not-found route

src/components/Header/index.tsx

@@ -44,7 +44,7 @@ export function Header({ navItems = [] }: { navItems?: any[] }) {
               <span className="font-heading font-bold text-lg leading-tight tracking-tight text-foreground">
                 Finanz- & Lohnbuchhaltung
               </span>
-              <span className="text-xs text-foreground/60 uppercase tracking-widest font-medium">
+              <span className="text-xs text-foreground/75 uppercase tracking-widest font-medium">
                 Dipl.-Ing. Janusz Kędzierski
               </span>
             </div>
@@ -57,7 +57,7 @@ export function Header({ navItems = [] }: { navItems?: any[] }) {
                 key={index} 
                 page={page}
                 currentLocaleCode={currentLocale.code}
-                className="text-sm font-medium text-foreground/80 hover:text-accent transition-colors"
+                className="text-sm font-medium text-foreground/90 hover:text-accent transition-colors"
               />
             ))}
           </nav>