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/shells/meldestelle-portal/build.gradle.kts

@@ -63,7 +63,7 @@ kotlin {
   sourceSets {
     commonMain.dependencies {
       // Shared modules
-      implementation(projects.frontend.shared)
+      // implementation(projects.frontend.shared) // REMOVED: Shared module deleted
       implementation(projects.frontend.core.domain)
       implementation(projects.frontend.core.designSystem)
       implementation(projects.frontend.core.navigation)