Improve error handling for Echo backend error format

- Update ErrorResponse model to make detail and statusCode optional since
  backend now returns simple {"error": "message"} format
- Update AuthApi to parse actual backend error messages instead of generic
  "Registration failed"/"Login failed" strings
- Update ErrorParser to prioritize the "error" field and add fallback for
  simple error map responses
- Update iOS ViewModels (Login, Register, AppleSignIn) to properly handle
  400 and 409 status codes by displaying backend error messages

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/com/example/casera/models/ErrorResponse.kt

@@ -3,10 +3,15 @@ package com.example.casera.models
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
+/**
+ * Error response model that handles the backend's error format.
+ * The backend returns: {"error": "message"}
+ * All fields except 'error' are optional for backwards compatibility.
+ */
 @Serializable
 data class ErrorResponse(
     val error: String,
-    val detail: String,
-    @SerialName("status_code") val statusCode: Int,
+    val detail: String? = null,
+    @SerialName("status_code") val statusCode: Int? = null,
     val errors: Map<String, List<String>>? = null
 )

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

@@ -19,7 +19,9 @@ class AuthApi(private val client: HttpClient = ApiClient.httpClient) {
             if (response.status.isSuccess()) {
                 ApiResult.Success(response.body())
             } else {
-                ApiResult.Error("Registration failed", response.status.value)
+                // Parse actual error message from backend
+                val errorMessage = ErrorParser.parseError(response)
+                ApiResult.Error(errorMessage, response.status.value)
             }
         } catch (e: Exception) {
             ApiResult.Error(e.message ?: "Unknown error occurred")
@@ -36,7 +38,9 @@ class AuthApi(private val client: HttpClient = ApiClient.httpClient) {
             if (response.status.isSuccess()) {
                 ApiResult.Success(response.body())
             } else {
-                ApiResult.Error("Login failed", response.status.value)
+                // Parse actual error message from backend
+                val errorMessage = ErrorParser.parseError(response)
+                ApiResult.Error(errorMessage, response.status.value)
             }
         } catch (e: Exception) {
             ApiResult.Error(e.message ?: "Unknown error occurred")

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

@@ -11,13 +11,27 @@ object ErrorParser {
         isLenient = true
     }
 
+    /**
+     * Parses error response from the backend.
+     * The backend returns: {"error": "message"}
+     * Falls back to detail field or field-specific errors if present.
+     */
     suspend fun parseError(response: HttpResponse): String {
         return try {
             val errorResponse = response.body<ErrorResponse>()
 
             // Build detailed error message
             val message = StringBuilder()
-            message.append(errorResponse.detail)
+
+            // Primary: use the error field (main error message from backend)
+            message.append(errorResponse.error)
+
+            // Secondary: append detail if present and different from error
+            errorResponse.detail?.let { detail ->
+                if (detail.isNotBlank() && detail != errorResponse.error) {
+                    message.append(": $detail")
+                }
+            }
 
             // Add field-specific errors if present
             errorResponse.errors?.let { fieldErrors ->
@@ -31,11 +45,18 @@ object ErrorParser {
 
             message.toString()
         } catch (e: Exception) {
-            // Fallback to reading as plain text
+            // Fallback: try to parse as simple {"error": "message"} map
             try {
-                response.body<String>()
-            } catch (e: Exception) {
-                "An error occurred (${response.status.value})"
+                val simpleError = response.body<Map<String, String>>()
+                simpleError["error"] ?: simpleError["message"] ?: simpleError["detail"]
+                    ?: "An error occurred (${response.status.value})"
+            } catch (e2: Exception) {
+                // Last resort: read as plain text
+                try {
+                    response.body<String>()
+                } catch (e3: Exception) {
+                    "An error occurred (${response.status.value})"
+                }
             }
         }
     }