refactor(auth, design-system): remove unused methods and annotations for cleanup

- Deleted obsolete methods such as `exchangeAuthorizationCode` and `logout` from the Auth module.
- Removed unused browser PKCE utilities and associated constants.
- Annotated unused components across the Design System with `@Suppress("unused")` for clarity.
- Simplified `LoginViewModel` to handle logout by clearing UI state directly.

frontend/shells/meldestelle-portal/src/commonMain/kotlin/MainApp.kt

@@ -4,20 +4,15 @@ import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.rememberScrollState
 import androidx.compose.foundation.verticalScroll
 import androidx.compose.ui.Modifier
-import androidx.compose.ui.platform.LocalUriHandler
 import androidx.compose.ui.unit.dp
 import androidx.compose.runtime.collectAsState
 import at.mocode.clients.shared.navigation.AppScreen
 import at.mocode.frontend.core.auth.data.AuthTokenManager
-import at.mocode.frontend.core.auth.data.AuthApiClient
 import at.mocode.frontend.core.auth.presentation.LoginScreen
 import at.mocode.frontend.core.auth.presentation.LoginViewModel
 import at.mocode.ping.feature.presentation.PingScreen
 import at.mocode.ping.feature.presentation.PingViewModel
-import at.mocode.shared.core.AppConstants
 import at.mocode.frontend.core.designsystem.components.AppFooter
-import kotlinx.coroutines.launch
-import androidx.compose.runtime.rememberCoroutineScope
 import org.koin.compose.koinInject
 import org.koin.compose.viewmodel.koinViewModel