fix(01-02): resolve compilation errors in auth models

- Fixed import path in AuthRepository to reach core errors
- Updated AuthUser.fromSupabase() to handle DateTime parsing safely
- Added UnknownAuthException for fallback error handling
- Fixed null-aware operators in AuthUser model
- All authentication files now compile without errors

lib/features/authentication/domain/repositories/auth_repository.dart

@@ -1,5 +1,5 @@
 import '../../data/models/auth_user.dart';
-import '../../../core/errors/auth_exceptions.dart';
+import '../../../../core/errors/auth_exceptions.dart';
 
 /// Authentication repository interface defining all authentication operations.
 ///