feat: add sales return functionality

- Introduced Sales Return types and interfaces in the frontend for managing sales returns.
- Implemented SalesReturnsController in the backend to handle sales return endpoints.
- Created SalesReturn and SalesReturnLine entities to represent sales return data.
- Added DTOs for sales return responses and requests.
- Configured Entity Framework for SalesReturn and SalesReturnLine entities.
- Developed ISalesReturnService interface and its implementation for business logic.
- Added API methods for listing sales returns, creating new returns, and fetching remaining returnable quantities.
- Created frontend components for creating and listing sales returns, including validation logic.
- Implemented UI for selecting invoices, reason codes, and managing return lines.

Frontend/erp-system/components/Layouts/AppSidebar.tsx

@@ -47,6 +47,7 @@ import {
   SlidersHorizontal,
   Tag,
   Truck,
+  Undo2,
   Users,
   Wallet,
   Warehouse,
@@ -120,6 +121,7 @@ const navItems: {
       // { title: "Reports", code: "sales.reports", href: "/dashboard/sales/reports", icon: FileBarChart },
     ],
   },
+  { title: "Sales Returns", code: "sales-returns", href: "/dashboard/sales/sales-returns", icon: Undo2 },
   { title: "Receiving", code: "receiving", href: "/dashboard/receiving/grn", icon: PackageCheck, chevron: true },
   {
     title: "Stock",
@@ -434,7 +436,7 @@ export function AppSidebar() {
   // seed grants it. This is also flagged in 02-SECURITY.md as the AR-09 sidebar-visibility
   // stopgap for HRM's salary/PII data — it hides HRM from the UI but doesn't enforce
   // anything server-side.
-  const bypassCodes = new Set(["procurement", "sales", "hrm", "production", "stock"])
+  const bypassCodes = new Set(["procurement", "sales", "sales-returns", "hrm", "production", "stock"])
   const visibleItems = loading
     ? []
     : navItems