feat: Implement fixed sale price functionality for items

- Added a toggle for fixed sale price vs stock value in the item creation form.
- Introduced validation to ensure all variants have a price greater than 0 when fixed price mode is selected.
- Updated the item model to include a nullable salePrice field, which is used for sales only and does not affect GRN/FIFO/ledger.
- Enhanced the GRN page to allow off-PO items and included a refresh button to update the item list without reloading the page.
- Updated documentation to reflect changes in item pricing and GRN handling.

docs/11-BACKEND-PHASE1.md

@@ -164,6 +164,8 @@ docs/10 C.9): every write below forwards to AuthHex's new `/api/role` functions
 
 ### 2.1 Items
 > **`itemType` → `stockNature` (2026-07-16).** The Stocked/NonStocked/Service field was renamed so the name `itemType` could be taken by the new Item Type master (§2.7) — an unrelated concept. Items gained `subCategoryId` and `brandId` (both nullable). Items carry **no** item-type reference: the values chosen in the builder are encoded into the client-generated SKU (docs/10 Part C.9).
+>
+> **`salePrice` added (nullable, 2026-07-22).** Every item body below carries `salePrice` (`number|null`). It is the **Sales-only** fixed selling price: `null` ⇒ the item is sold at its stock/FIFO value; a value ⇒ fixed price. It never affects GRN/FIFO/ledger. On write it is optional; when supplied it must be `>= 0` (else `400` validation). The item builder's "fixed price / use stock value" toggle is UI-only — the contract is just the nullable field.
 
 #### `GET /items`
 Query: `q`, `status` (`Active|Inactive`), `categoryId`, `subCategoryId`, `brandId`, `trackingMode` (`None|Batch|Serial`), + paging.
@@ -171,7 +173,8 @@ Query: `q`, `status` (`Active|Inactive`), `categoryId`, `subCategoryId`, `brandI
 ```json
 { "items": [ { "itemId": 1001, "sku": "ITM-1001", "name": "Steel Bolt M8x40",
       "categoryId": 12, "subCategoryId": 30, "brandId": 2, "baseUomId": 1, "defaultVendorId": 5,
-      "stockNature": "Stocked", "trackingMode": "Batch", "taxClass": "STD", "status": "Active" } ],
+      "stockNature": "Stocked", "trackingMode": "Batch", "taxClass": "STD",
+      "salePrice": 12.5000, "status": "Active" } ],
   "pagination": { "page": 1, "pageSize": 20, "totalItems": 1, "totalPages": 1 } }
 ```
 
@@ -181,7 +184,8 @@ Query: `q`, `status` (`Active|Inactive`), `categoryId`, `subCategoryId`, `brandI
   "description": "Grade 8.8 zinc-plated hex bolt", "categoryId": 12, "subCategoryId": 30,
   "brandId": 2, "baseUomId": 1,
   "defaultVendorId": 5, "stockNature": "Stocked", "trackingMode": "Batch", "taxClass": "STD",
-  "status": "Active", "reorder": [ { "warehouseId": 1, "reorderPoint": 500, "reorderQty": 2000 } ],
+  "salePrice": 12.5000, "status": "Active",
+  "reorder": [ { "warehouseId": 1, "reorderPoint": 500, "reorderQty": 2000 } ],
   "conversions": [ { "conversionId": 33, "fromUom": 7, "toUom": 1, "factor": 12 } ],
   "createdAt": "2026-06-01T08:00:00Z", "updatedAt": "2026-07-01T10:15:00Z" }
 ```
@@ -192,14 +196,14 @@ The `sku` is **generated by the client** (it encodes the chosen item-type values
 ```json
 { "sku": "ITM-1002", "name": "Steel Nut M8", "description": "Grade 8 zinc-plated hex nut",
   "categoryId": 12, "subCategoryId": 30, "brandId": 2, "baseUomId": 1, "defaultVendorId": 5,
-  "stockNature": "Stocked", "trackingMode": "None", "taxClass": "STD" }
+  "stockNature": "Stocked", "trackingMode": "None", "taxClass": "STD", "salePrice": 3.2500 }
 ```
 **201 Created** — `Location: /api/v1/items/1002`
 ```json
 { "itemId": 1002, "sku": "ITM-1002", "name": "Steel Nut M8", "categoryId": 12,
   "subCategoryId": 30, "brandId": 2, "baseUomId": 1,
   "defaultVendorId": 5, "stockNature": "Stocked", "trackingMode": "None", "taxClass": "STD",
-  "status": "Active", "createdAt": "2026-07-07T09:30:00Z" }
+  "salePrice": 3.2500, "status": "Active", "createdAt": "2026-07-07T09:30:00Z" }
 ```
 `400` → `code: SKU_DUPLICATE` if SKU exists.
 `422` → `code: CONFIG_DISABLED` if `subCategoryId` is sent while subcategories are disabled, or `brandId` while brands are disabled (§2.8).
@@ -530,11 +534,18 @@ Against a PO (lines default from open PO lines) or direct (`poId: null`, by perm
 { "poId": 342, "warehouseId": 1,
   "lines": [ { "poLineId": 900, "itemId": 1001, "uomId": 1, "binId": 45, "qty": 5000,
       "unitCost": 12.50, "discountPct": 10, "vatPct": 18, "holdStatus": "OnHold",
-      "batch": { "batchNo": "B-2607", "expiryDate": "2028-07-01" } } ] }
+      "batch": { "batchNo": "B-2607", "expiryDate": "2028-07-01" } },
+    { "poLineId": null, "itemId": 1050, "uomId": 1, "qty": 20,
+      "unitCost": 8.00, "holdStatus": "Available" } ] }
 ```
 `discountPct`/`vatPct` optional (default 0, range 0–100). `unitCost` on a **PO line** is an optional
 override: 0/omitted uses the PO price; a value wins and a variance is recorded (02-SECURITY C.3, revised).
 On a direct receipt `unitCost` is required.
+**Off-PO lines (`poLineId: null`) are allowed even on a PO-based GRN** (2026-07-22, FR-GRN-01) — the second
+line above receives an item that is not on the PO. Such a line behaves exactly like a direct-receipt line:
+`unitCost` is required, `OVER_RECEIPT_TOLERANCE` does **not** apply (there is no PO qty to check), and no PO
+line balance is touched. The inline "create new item" UI simply calls `POST /items` (§2.1) first, then adds
+the returned item as an off-PO line.
 **201 Created** — status `Draft`. All derived figures are **server-computed**:
 `netUnitCost = unitCost × (1 − discountPct/100)`, `receivedValue = qty × netUnitCost` (after discount,
 **before** VAT — this is the stock value), `vatAmount = receivedValue × vatPct/100`,