feat(grn): add update functionality for draft GRNs and support document-level discounts

- Implemented Update method in GrnsController to allow editing of draft GRNs.
- Enhanced CreateGrnRequest to include an optional totalDiscount property.
- Updated GrnService to handle GRN updates, including validation and line item processing.
- Modified NewGrnPage to support editing existing GRNs and applying document-level discounts.
- Improved UI in NewItemPage and GrnDetailPage for better user experience.
- Added search functionality to Select component for improved item selection.

Frontend/erp-system/lib/api/grns.ts

@@ -43,6 +43,10 @@ export const grnsApi = {
     return apiRequest<Grn>("/grns", { method: "POST", body: request })
   },
 
+  update(grnId: number, request: CreateGrnRequest): Promise<Grn> {
+    return apiRequest<Grn>(`/grns/${grnId}`, { method: "PUT", body: request })
+  },
+
   /**
    * Posts the receipt. Pass a stable idempotencyKey per detail-page session so a retry
    * cannot double-post stock — unlike the old mock, the server genuinely dedupes on it.