refactor(auth): migrate Auth module from feature to core package

- Reorganized `auth-feature` into `core/auth` to improve architecture and modularity.
- Removed unused PKCE and OAuth callback utilities (`AuthCallbackParams`, `OAuthPkceService`).
- Updated imports and adjusted build scripts to reflect new module structure.
- Refactored `LoginScreen` and `PingScreen` to include `onBack` functionality in top bars for improved navigation.
- Corrected sync endpoint in `PingSyncService`.

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

@@ -80,8 +80,8 @@ kotlin {
       implementation(projects.frontend.core.navigation)
       implementation(projects.frontend.core.network)
       implementation(projects.frontend.core.sync)
-      implementation(project(":frontend:core:local-db"))
-      implementation(projects.frontend.features.authFeature)
+      implementation(projects.frontend.core.localDb)
+      implementation(projects.frontend.core.auth)
       implementation(projects.frontend.features.pingFeature)
 
       // DI (Koin) needed to call initKoin { modules(...) }