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/core/errors/auth_exceptions.dart

@@ -146,6 +146,19 @@ class AuthDisabledException extends AuthException {
         );
 }
 
+/// Exception thrown for unknown authentication errors
+class UnknownAuthException extends AuthException {
+  const UnknownAuthException({
+    String message = 'An unknown authentication error occurred',
+    String? code,
+    dynamic originalError,
+  }) : super(
+          message: message,
+          code: code ?? 'UNKNOWN_ERROR',
+          originalError: originalError,
+        );
+}
+
 /// Utility class for converting Supabase errors to custom exceptions
 /// 
 /// Provides a centralized way to map Supabase error responses
@@ -263,7 +276,7 @@ class AuthExceptionFactory {
     }
     
     // Fallback for unknown errors
-    return AuthException(
+    return UnknownAuthException(
       message: _extractUserFriendlyMessage(errorMessage) ?? 'Authentication failed',
       code: errorCode,
       originalError: error,