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>

iosApp/iosApp/Login/AppleSignInViewModel.swift

@@ -125,12 +125,16 @@ class AppleSignInViewModel: ObservableObject {
 
         if let code = error.code?.intValue {
             switch code {
-            case 400:
-                errorMessage = "Invalid Apple Sign In token"
             case 401:
                 errorMessage = "Authentication failed. Please try again."
             case 403:
                 errorMessage = "Access denied"
+            case 409:
+                // Conflict - let backend message explain the issue
+                errorMessage = ErrorMessageParser.parse(error.message)
+            case 400:
+                // Bad request - validation errors from backend
+                errorMessage = ErrorMessageParser.parse(error.message)
             case 500...599:
                 errorMessage = "Server error. Please try again later."
             default:

iosApp/iosApp/Login/LoginViewModel.swift

@@ -106,12 +106,18 @@ class LoginViewModel: ObservableObject {
         // Check for specific error codes and provide user-friendly messages
         if let code = error.code?.intValue {
             switch code {
-            case 400, 401:
+            case 401:
                 self.errorMessage = "Invalid username or password"
             case 403:
                 self.errorMessage = "Access denied. Please check your credentials."
             case 404:
                 self.errorMessage = "Service not found. Please try again later."
+            case 409:
+                // Conflict - let backend message explain the issue
+                self.errorMessage = ErrorMessageParser.parse(error.message)
+            case 400:
+                // Bad request - validation errors from backend
+                self.errorMessage = ErrorMessageParser.parse(error.message)
             case 500...599:
                 self.errorMessage = "Server error. Please try again later."
             default:

iosApp/iosApp/Register/RegisterViewModel.swift

@@ -72,7 +72,24 @@ class RegisterViewModel: ObservableObject {
                     self.isRegistered = true
                     self.isLoading = false
                 } else if let error = result as? ApiResultError {
-                    self.errorMessage = ErrorMessageParser.parse(error.message)
+                    // Handle specific HTTP status codes
+                    if let code = error.code?.intValue {
+                        switch code {
+                        case 409:
+                            // Conflict - duplicate username or email
+                            // The backend error message already contains the specific conflict
+                            self.errorMessage = ErrorMessageParser.parse(error.message)
+                        case 400:
+                            // Bad request - validation errors
+                            self.errorMessage = ErrorMessageParser.parse(error.message)
+                        case 500...599:
+                            self.errorMessage = "Server error. Please try again later."
+                        default:
+                            self.errorMessage = ErrorMessageParser.parse(error.message)
+                        }
+                    } else {
+                        self.errorMessage = ErrorMessageParser.parse(error.message)
+                    }
                     self.isLoading = false
                 }
             } catch {