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`.

settings.gradle.kts

@@ -129,6 +129,8 @@ include(":docs")
 // FRONTEND
 // ==========================================================================
 // --- CORE ---
+// frontend/core/auth
+include(":frontend:core:auth") // MOVED from features
 include(":frontend:core:domain")
 include(":frontend:core:design-system")
 include(":frontend:core:navigation")
@@ -137,7 +139,6 @@ include(":frontend:core:local-db")
 include(":frontend:core:sync")
 
 // --- FEATURES ---
-include(":frontend:features:auth-feature")
 // include(":frontend:features:members-feature")
 include(":frontend:features:ping-feature")