Add Sign in with Apple for iOS

Kotlin Shared Layer:
- Add AppleSignInRequest and AppleSignInResponse models
- Add appleSignIn method to AuthApi and APILayer
- Add appleSignInState and appleSignIn() to AuthViewModel

iOS App:
- Create AppleSignInManager for AuthenticationServices integration
- Create AppleSignInViewModel to coordinate Apple auth flow
- Update LoginView with "Sign in with Apple" button
- Add Sign in with Apple entitlement
- Add accessibility identifier for UI testing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/com/example/casera/viewmodel/AuthViewModel.kt

@@ -2,6 +2,8 @@ package com.example.casera.viewmodel
 
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
+import com.example.casera.models.AppleSignInRequest
+import com.example.casera.models.AppleSignInResponse
 import com.example.casera.models.AuthResponse
 import com.example.casera.models.ForgotPasswordRequest
 import com.example.casera.models.ForgotPasswordResponse
@@ -48,6 +50,9 @@ class AuthViewModel : ViewModel() {
     private val _resetPasswordState = MutableStateFlow<ApiResult<ResetPasswordResponse>>(ApiResult.Idle)
     val resetPasswordState: StateFlow<ApiResult<ResetPasswordResponse>> = _resetPasswordState
 
+    private val _appleSignInState = MutableStateFlow<ApiResult<AppleSignInResponse>>(ApiResult.Idle)
+    val appleSignInState: StateFlow<ApiResult<AppleSignInResponse>> = _appleSignInState
+
     fun login(username: String, password: String) {
         viewModelScope.launch {
             _loginState.value = ApiResult.Loading
@@ -207,6 +212,36 @@ class AuthViewModel : ViewModel() {
         _resetPasswordState.value = ApiResult.Idle
     }
 
+    fun appleSignIn(
+        idToken: String,
+        userId: String,
+        email: String?,
+        firstName: String?,
+        lastName: String?
+    ) {
+        viewModelScope.launch {
+            _appleSignInState.value = ApiResult.Loading
+            val result = APILayer.appleSignIn(
+                AppleSignInRequest(
+                    idToken = idToken,
+                    userId = userId,
+                    email = email,
+                    firstName = firstName,
+                    lastName = lastName
+                )
+            )
+            _appleSignInState.value = when (result) {
+                is ApiResult.Success -> ApiResult.Success(result.data)
+                is ApiResult.Error -> result
+                else -> ApiResult.Error("Unknown error")
+            }
+        }
+    }
+
+    fun resetAppleSignInState() {
+        _appleSignInState.value = ApiResult.Idle
+    }
+
     fun logout() {
         viewModelScope.launch {
             APILayer.logout()