Add comprehensive error message parsing to prevent raw JSON display

- Created ErrorMessageParser utility for both iOS (Swift) and Android (Kotlin)
- Parser detects JSON-formatted error messages and extracts user-friendly text
- Identifies when data objects (not errors) are returned and provides generic messages
- Updated all API error handling to pass raw error bodies instead of concatenating
- Applied ErrorMessageParser across all ViewModels and screens on both platforms
- Fixed ContractorApi and DocumentApi to not concatenate error bodies with messages
- Updated ApiResultHandler to automatically parse all error messages
- Error messages now show "Request failed. Please check your input and try again." instead of raw JSON

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

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

iosApp/iosApp/Register/RegisterView.swift

@@ -127,6 +127,10 @@ struct RegisterView: View {
                     }
                 )
             }
+            .handleErrors(
+                error: viewModel.errorMessage,
+                onRetry: { viewModel.register() }
+            )
         }
     }
 }