chore(infra): Network/Auth – DoD schließen

- Entfernen/Deprecaten: `frontend/features/auth-feature/.../AuthenticatedHttpClient.kt` und alle manuellen `Authorization`‑Header‑Setzungen.
              - Stattdessen: DI‑`apiClient` via Koin injizieren (`single(named("apiClient"))`) und Token‑Anreicherung über Ktor `Auth` Plugin (Bearer) verdrahten.
              - Build‑Guard ergänzen: Auch Vorkommen von `HttpHeaders.Authorization` erkennen.

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

@@ -6,8 +6,8 @@ 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.clients.authfeature.AuthenticatedHttpClient
 import at.mocode.clients.authfeature.AuthTokenManager
+import org.koin.core.context.GlobalContext
 import at.mocode.clients.pingfeature.PingScreen
 import at.mocode.clients.pingfeature.PingViewModel
 import at.mocode.shared.core.AppConstants
@@ -29,7 +29,8 @@ fun MainApp() {
     ) {
       var currentScreen by remember { mutableStateOf<AppScreen>(AppScreen.Home) }
 
-      val authTokenManager = remember { AuthenticatedHttpClient.getAuthTokenManager() }
+      // Resolve AuthTokenManager from Koin
+      val authTokenManager = remember { GlobalContext.get().koin.get<AuthTokenManager>() }
       val pingViewModel = remember { PingViewModel() }
       val scope = rememberCoroutineScope()