Add Apple Login => Works only on iPhone for now, will be adding rest later.

lib/services/auth_service.dart

@@ -1,6 +1,7 @@
 import 'package:firebase_auth/firebase_auth.dart';
 import 'package:google_sign_in_platform_interface/google_sign_in_platform_interface.dart';
 import 'package:travel_mate/services/error_service.dart';
+import 'package:sign_in_with_apple/sign_in_with_apple.dart';
 
 /// Service for handling Firebase authentication operations.
 ///
@@ -178,8 +179,83 @@ class AuthService {
 
   /// Signs in a user using Apple authentication.
   ///
-  /// This method is currently a placeholder for future Apple authentication support.
-  Future signInWithApple() async {
-    // TODO: Implement Apple sign-in
+  /// Handles the complete Apple Sign-In flow including platform initialization
+  /// and credential exchange with Firebase. Supports both iOS and Android platforms.
+  ///
+  /// Returns a [UserCredential] containing the authenticated user's information.
+  /// Throws various exceptions if authentication fails or platform is not supported.
+  Future<UserCredential> signInWithApple() async {
+    try {
+      await ensureInitialized();
+
+      // Check if Apple Sign-In is available on this platform
+      final bool isAvailable = await SignInWithApple.isAvailable();
+      if (!isAvailable) {
+        throw FirebaseAuthException(
+          code: 'ERROR_APPLE_SIGNIN_NOT_AVAILABLE',
+          message: 'Apple Sign-In is not available on this platform',
+        );
+      }
+
+      // Request Apple ID credential with platform-specific configuration
+      final AuthorizationCredentialAppleID credential =
+          await SignInWithApple.getAppleIDCredential(
+            scopes: [
+              AppleIDAuthorizationScopes.email,
+              AppleIDAuthorizationScopes.fullName,
+            ],
+            // Configuration for Android/Web
+            webAuthenticationOptions: WebAuthenticationOptions(
+              clientId: 'be.devdayronvl.TravelMate',
+              redirectUri: Uri.parse(
+                'https://your-project-id.firebaseapp.com/__/auth/handler',
+              ),
+            ),
+          );
+
+      // Create OAuth credential for Firebase
+      final OAuthCredential oauthCredential = OAuthProvider("apple.com")
+          .credential(
+            idToken: credential.identityToken,
+            accessToken: credential.authorizationCode,
+          );
+
+      // Sign in with Firebase
+      UserCredential userCredential = await firebaseAuth.signInWithCredential(
+        oauthCredential,
+      );
+
+      // Update display name if it's the first sign-in and we have name info
+      if (userCredential.additionalUserInfo?.isNewUser == true &&
+          credential.givenName != null &&
+          credential.familyName != null) {
+        final String displayName =
+            '${credential.givenName} ${credential.familyName}';
+        await userCredential.user?.updateDisplayName(displayName);
+      }
+
+      return userCredential;
+    } on SignInWithAppleException catch (e) {
+      _errorService.logError('Apple Sign-In error: $e', StackTrace.current);
+      throw FirebaseAuthException(
+        code: 'ERROR_APPLE_SIGNIN_FAILED',
+        message: 'Apple Sign-In failed: ${e.toString()}',
+      );
+    } on FirebaseAuthException catch (e) {
+      _errorService.logError(
+        'Firebase error during Apple Sign-In: $e',
+        StackTrace.current,
+      );
+      rethrow;
+    } catch (e) {
+      _errorService.logError(
+        'Unknown error during Apple Sign-In: $e',
+        StackTrace.current,
+      );
+      throw FirebaseAuthException(
+        code: 'ERROR_APPLE_SIGNIN_UNKNOWN',
+        message: 'Unknown error during Apple Sign-In: $e',
+      );
+    }
   }
 }