Add vendor selection and rates to order creation flow

Introduces a Vendor entity and integrates vendor selection into the one-time order creation and order editing flows. Vendor-specific rates are now displayed and used for position roles, and UI components have been updated to allow users to select a vendor and see corresponding rates. Mock vendor data is used for demonstration purposes.

apps/mobile/packages/domain/lib/src/entities/business/vendor.dart

@@ -0,0 +1,15 @@
+import 'package:equatable/equatable.dart';
+
+/// Represents a staffing vendor.
+class Vendor extends Equatable {
+  const Vendor({required this.id, required this.name, required this.rates});
+
+  final String id;
+  final String name;
+
+  /// A map of role names to hourly rates.
+  final Map<String, double> rates;
+
+  @override
+  List<Object?> get props => <Object?>[id, name, rates];
+}