feat: replace mzur/kirby-uniform with custom Kontaktformular implementation

Replaced the `mzur/kirby-uniform` dependency with a custom form handling solution. Updated `kontakt.php`, its backend controller, and corresponding email template for streamlined submission handling. Adjusted styles in `input.css` and integrated a honeypot field for spam prevention.

src/css/input.css

@@ -70,7 +70,7 @@
         @apply text-sf_blau-500 font-bold hover:text-sf_gelb-600;
     }
 
-    .uniform__potty {
+    .honeypot {
         position: absolute;
         left: -9999px;
     }