feat(desktop): add NennungsEingang screen and integrate into navigation

- Introduced `NennungsEingangScreen` for managing online nomination entries.
- Added `NennungsEingang` to `AppScreen` with corresponding route configuration.
- Updated `DesktopMainLayout` to include navigation and UI components for `NennungsEingang`.
- Adjusted `PreviewMain` for screen integration and testing.

frontend/core/navigation/src/commonMain/kotlin/at/mocode/frontend/core/navigation/AppScreen.kt

@@ -65,6 +65,7 @@ sealed class AppScreen(val route: String) {
   data object Meisterschaften : AppScreen("/meisterschaften")
   data object Cups : AppScreen("/cups")
   data object StammdatenImport : AppScreen("/stammdaten/import")
+  data object NennungsEingang : AppScreen("/nennungs-eingang")
 
   companion object {
     private val VERANSTALTUNG_DETAIL = Regex("/veranstaltung/(\\d+)$")
@@ -106,6 +107,7 @@ sealed class AppScreen(val route: String) {
         "/meisterschaften" -> Meisterschaften
         "/cups" -> Cups
         "/stammdaten/import" -> StammdatenImport
+        "/nennungs-eingang" -> NennungsEingang
         else -> {
           BILLING.matchEntire(route)?.destructured?.let { (vId, tId) ->
             return Billing(vId.toLong(), tId.toLong())