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/jvmMain/kotlin/main.kt

@@ -4,12 +4,13 @@ import androidx.compose.ui.window.WindowState
 import androidx.compose.ui.unit.dp
 import at.mocode.shared.di.initKoin
 import at.mocode.frontend.core.network.networkModule
+import at.mocode.clients.authfeature.di.authFeatureModule
 
 fun main() = application {
   // Initialize DI (Koin) with shared modules + network module
   try {
-    initKoin { modules(networkModule) }
-    println("[DesktopApp] Koin initialized with networkModule")
+    initKoin { modules(networkModule, authFeatureModule) }
+    println("[DesktopApp] Koin initialized with networkModule + authFeatureModule")
   } catch (e: Exception) {
     println("[DesktopApp] Koin initialization warning: ${e.message}")
   }