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/models/User.kt

@@ -162,3 +162,27 @@ data class ResetPasswordResponse(
 data class MessageResponse(
     val message: String
 )
+
+// Apple Sign In Models
+
+/**
+ * Apple Sign In request matching Go API
+ */
+@Serializable
+data class AppleSignInRequest(
+    @SerialName("id_token") val idToken: String,
+    @SerialName("user_id") val userId: String,
+    val email: String? = null,
+    @SerialName("first_name") val firstName: String? = null,
+    @SerialName("last_name") val lastName: String? = null
+)
+
+/**
+ * Apple Sign In response matching Go API
+ */
+@Serializable
+data class AppleSignInResponse(
+    val token: String,
+    val user: User,
+    @SerialName("is_new_user") val isNewUser: Boolean
+)

composeApp/src/commonMain/kotlin/com/example/casera/network/APILayer.kt

@@ -921,6 +921,20 @@ object APILayer {
         return authApi.resetPassword(request)
     }
 
+    suspend fun appleSignIn(request: AppleSignInRequest): ApiResult<AppleSignInResponse> {
+        val result = authApi.appleSignIn(request)
+
+        // Update cache on success
+        if (result is ApiResult.Success) {
+            TokenStorage.saveToken(result.data.token)
+            DataCache.updateCurrentUser(result.data.user)
+            // Prefetch all data after successful Apple sign in
+            prefetchManager.prefetchAllData()
+        }
+
+        return result
+    }
+
     suspend fun updateProfile(token: String, request: UpdateProfileRequest): ApiResult<User> {
         val result = authApi.updateProfile(token, request)
 

composeApp/src/commonMain/kotlin/com/example/casera/network/AuthApi.kt

@@ -200,4 +200,27 @@ class AuthApi(private val client: HttpClient = ApiClient.httpClient) {
             ApiResult.Error(e.message ?: "Unknown error occurred")
         }
     }
+
+    // Apple Sign In
+    suspend fun appleSignIn(request: AppleSignInRequest): ApiResult<AppleSignInResponse> {
+        return try {
+            val response = client.post("$baseUrl/auth/apple-sign-in/") {
+                contentType(ContentType.Application.Json)
+                setBody(request)
+            }
+
+            if (response.status.isSuccess()) {
+                ApiResult.Success(response.body())
+            } else {
+                val errorBody = try {
+                    response.body<Map<String, String>>()
+                } catch (e: Exception) {
+                    mapOf("error" to "Apple Sign In failed")
+                }
+                ApiResult.Error(errorBody["error"] ?: "Apple Sign In failed", response.status.value)
+            }
+        } catch (e: Exception) {
+            ApiResult.Error(e.message ?: "Unknown error occurred")
+        }
+    }
 }

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()