chore(frontend): refactor navigation and DI setup, remove unused shared module

- Replaced `initKoin` with `startKoin` for DI initialization consistency across platforms.
- Introduced `StateNavigationPort` with `StateFlow` to streamline navigation state management.
- Migrated `AppScreen` to sealed class with route mapping for better navigation handling.
- Deleted unused `frontend/shared` module and removed related dependencies from build files.
- Cleaned up legacy navigation and Redux-related code, aligning with MVVM architecture.

frontend/core/domain/src/commonMain/kotlin/at/mocode/frontend/core/domain/AppConstants.kt

@@ -0,0 +1,11 @@
+package at.mocode.frontend.core.domain
+
+object AppConstants {
+  // Keycloak Configuration
+  // Note: These defaults are for local development.
+  // In production, these should be provided via build config or environment variables.
+  const val KEYCLOAK_URL = "http://localhost:8180"
+  const val KEYCLOAK_REALM = "meldestelle"
+  const val KEYCLOAK_CLIENT_ID = "web-app"
+  const val KEYCLOAK_CLIENT_SECRET = "" // Public client usually has no secret
+}