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/data/models/auth_user.dart

@@ -45,11 +45,11 @@ class AuthUser {
     return AuthUser(
       id: user.id,
       email: user.email ?? '',
-      createdAt: user.createdAt ?? DateTime.now(),
+      createdAt: DateTime.tryParse(user.createdAt.toString()) ?? DateTime.now(),
       emailVerified: user.emailConfirmedAt != null,
       displayName: user.userMetadata?['display_name'] as String?,
       avatarUrl: user.userMetadata?['avatar_url'] as String?,
-      lastSignInAt: user.lastSignInAt,
+      lastSignInAt: DateTime.tryParse(user.lastSignInAt.toString()),
     );
   }