refactor(ui, navigation): implement platform-specific routing and redesign components

- Added platform detection logic `currentPlatform()` in `PlatformType.js.kt`.
- Introduced platform-based behavior for LandingScreen, Dashboard, and Login flow.
- Replaced Row with FlowRow in PingScreen to improve button layout.
- Updated Meldestelle Dashboard with platform-specific headers and authentication checks.
- Adjusted AppHeader to accept `isAuthenticated` and `username` parameters.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

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

@@ -5,7 +5,10 @@ sealed class AppScreen(val route: String) {
   data object Home : AppScreen("/home")
   data object Dashboard : AppScreen("/dashboard")
   data object CreateTournament : AppScreen("/tournament/create") // Neuer Screen
-  data object Login : AppScreen(Routes.LOGIN)
+
+  // Login now accepts an optional returnTo screen to determine where to go after success
+  data class Login(val returnTo: AppScreen? = null) : AppScreen(Routes.LOGIN)
+
   data object Ping : AppScreen("/ping")
   data object Profile : AppScreen("/profile")
   data object AuthCallback : AppScreen("/auth/callback")
@@ -17,7 +20,7 @@ sealed class AppScreen(val route: String) {
         "/home" -> Home
         "/dashboard" -> Dashboard
         "/tournament/create" -> CreateTournament
-        Routes.LOGIN, Routes.Auth.LOGIN -> Login
+        Routes.LOGIN, Routes.Auth.LOGIN -> Login()
         "/ping" -> Ping
         "/profile" -> Profile
         "/auth/callback" -> AuthCallback