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/lib/validations/sales.ts

@@ -0,0 +1,18 @@
+// Client-side UX validation only — required fields, format/range checks the
+// browser can already see. Server-authoritative rules (referential existence,
+// concurrency, reason-code context) are never re-implemented here. Same
+// pattern as lib/validations/procurement.ts's validateReturnLine.
+
+export function validateSalesReturnLine(input: { salesInvoiceLineId: number | null; qty: string; maxQty: number | null }): Record<string, string> {
+  const errors: Record<string, string> = {}
+  if (!input.salesInvoiceLineId) errors.salesInvoiceLineId = "Select an invoiced line"
+  const qty = Number(input.qty)
+  if (!input.qty || Number.isNaN(qty) || qty <= 0) errors.qty = "Quantity must be greater than 0"
+  // Client-side sanity bound on the remaining returnable qty — the server remains authoritative.
+  if (input.maxQty !== null && qty > input.maxQty) {
+    errors.qty = input.maxQty <= 0
+      ? "This line has already been fully returned"
+      : `Insufficient quantity — only ${input.maxQty} remain returnable`
+  }
+  return errors
+}