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.

Backend/PROGRESS.md

@@ -27,6 +27,7 @@ Convention: `docs/01-DOC-GUIDE.md §6`. Update this file in the **same commit**
 - [x] Item Type master (FR-MD-10) — CRUD + status + ETag; **unreferenced by design**, feeds the builder dropdown only
 - [x] SubCategory (FR-MD-04) — nested list/create under a category, `PUT`/`PATCH status` by id; `Item.subCategoryId` nullable FK, validated to belong to `categoryId`
 - [x] Product Configuration (FR-MD-11) — singleton `GET`/`PUT /product-config`; `CONFIG_DISABLED` gating on item writes
+- [x] Item **sale price** (FR-MD-01, 2026-07-22) — nullable `Item.SalePrice` (`numeric(18,4)`); on all Item DTOs (list/detail/create/update), validated `>= 0`. **Sales-only** — never enters GRN/FIFO/ledger. `null` ⇒ sell at stock value. Migration `AddItemSalePrice`. See the 2026-07-22 Done entry.
 
 > ### 2026-07-16 — Brands, Subcategories, Item Types, Product Config (migration #2)
 > Makes real three concepts the frontend had been faking on mock data (`Frontend/erp-system/lib/api/mock-data.ts`), per docs/10 §B.3.1 FR-MD-09/10/11 and docs/11 §2.3/2.6/2.7/2.8.
@@ -75,6 +76,7 @@ Convention: `docs/01-DOC-GUIDE.md §6`. Update this file in the **same commit**
 ## 3. Goods Receipt
 > Implemented + **smoke test PASSED** 2026-07-13 (see §4 note for the shared stock verification). Same `[~]` reason as §1/§2: the §6 auth+audit gate.
 - [x] GRN create (against PO / direct), over-receipt tolerance — `unitCost` **defaults to the PO price but is now overridable per line** (variance recorded vs `poUnitPrice` snapshot — 02-SECURITY C.3 revised 2026-07-20; the old "client cost ignored" block is gone); direct receipt requires `vendorId` + entered cost (AR-04); over-receipt → `422 OVER_RECEIPT_TOLERANCE` (verified at open-qty boundary); batch created/reused per (item, batchNo). Serial capture deferred. **Discount/VAT added** — see the 2026-07-20 entry.
+- [x] **Off-PO lines on a PO-based GRN** (FR-GRN-01, 2026-07-22) — a line with `poLineId: null` on a PO-based GRN is received like a direct line (entered `unitCost`, no over-receipt check, PO balances untouched). **No code change was needed** — `GrnService.CreateAsync` already branches per-line on `input.PoLineId is not null`; documented + frontend-enabled. Same review/audit surface as AR-04 (02-SECURITY C.3).
 - [x] GRN confirm → FIFO layer + ledger + PO `qtyReceived` (single UoW txn) — verified: layers+ledger posted, running balance, PO → PartiallyReceived/FullyReceived, UOM→base conversion (10 Box-12 → 120 base @10). **Idempotent** re-confirm verified (no double-post). Note: `Idempotency-Key` accepted but idempotency is resource-state based (already-Confirmed replays existing result); a keyed idempotency store is deferred.
 - [x] Inspection hold release / reject — Release fully verified (OnHold excluded from `available`, then released). Reject removes on-hand + posts a reversing ledger entry; formal link to a Purchase Return is deferred (§3.4).
 
@@ -111,7 +113,11 @@ Convention: `docs/01-DOC-GUIDE.md §6`. Update this file in the **same commit**
 ## Done
 <!-- move [x] items here with date + note if the active list grows long -->
 
-### 2026-07-20 — PO draft lifecycle + GRN discount/VAT/price-override (migration `AddGrnPricingAndPoDraft`)
+### 2026-07-22 — Item fixed sale price + GRN off-PO items (migration `AddItemSalePrice`)
+- **Item sale price (FR-MD-01).** New nullable `Item.SalePrice` (`numeric(18,4)`, `ItemConfiguration.HasPrecision(18,4)`), threaded through `ItemListItemDto`/`ItemDetailDto`/`CreateItemRequest`/`UpdateItemRequest` (`[Range(0, …)]`) and mapped in `ItemService` (create/update/`ToDetail`/list projection). **Sales-only** — it never touches `GrnService`, FIFO, `StockLayer`, or the ledger, so receipt/costing behaviour is byte-for-byte unchanged. `null` ⇒ "use stock value"; the fixed-vs-stock choice is a frontend toggle, not a server field (no `price_mode` enum). docs/10 C.1/C.9 + decision #14, docs/11 §2.1, 02-SECURITY C.1.
+- **GRN off-PO items (FR-GRN-01).** A PO-based GRN may now carry lines with `poLineId: null` (item not on the PO). **No backend change** — `GrnService.CreateAsync` already routed such lines through the direct-receipt path (entered cost, no over-receipt check, no PO-balance update). Documented as intended behaviour; the frontend now exposes it. docs/10 FR-GRN-01/C.3 (`po_line_id` nullable) + decision #15, docs/11 §4.1, 02-SECURITY C.3.
+- **Migration** `AddItemSalePrice` — single nullable column add; no backfill (`Down()` drops it). **Applied** to the local DB (`dotnet ef database update` → Done).
+- **Verified:** `dotnet build` clean (compile succeeded; the only earlier failure was the running dev exe holding a file lock, resolved by stopping it). Migration Up applied. Frontend `tsc --noEmit` clean. End-to-end runtime smoke (Swagger/UI) still to be run by the user.
 - **PO draft lifecycle (FR-PROC-05 revised).** `CreatePurchaseOrderRequest.SaveAsDraft` (default `false` → auto-approve unchanged; `true` → `Draft`). New `POST /purchase-orders/{id}/submit` (Draft→Approved, else `409 PO_NOT_EDITABLE`) and `DELETE /purchase-orders/{id}` (Draft-only, else 409). `IsEditable` narrowed from "not FullyReceived/Closed/Cancelled" to **`Draft` only** — so `PUT` now 409s on any submitted PO. **Option B ("freely edit while open") is superseded**; docs/10 FR-PROC-05, docs/11 §3.3 updated. ⚠️ **Every pre-existing PO is `Approved` and therefore now uneditable/undeletable** — intended, not a regression. No schema change (reuses the existing `Draft` enum value).
 - **GRN discount + VAT + price override.** `GrnLine` gained `PoUnitPrice`(nullable snapshot), `DiscountPct`, `NetUnitCost`, `VatPct`, `VatAmount`, `LineTotal`. All derived figures **server-computed**, never client-supplied. FIFO layer + ledger now cost at **`NetUnitCost`** (after discount) — VAT is recoverable and never enters stock value (docs/10 FR-GRN-06 revised). For a PO line, `unitCost` defaults to the PO price but an entered override wins and a **variance** is recorded against `PoUnitPrice` (02-SECURITY C.3 revised — the "client cost ignored, decision locked" control is deliberately loosened; the variance trail + audit log are the compensating control). Multi-GRN-per-PO at differing prices (the 20/50/30 case) already worked via `openQty`/`QtyReceived` and is untouched.
 - **Migration** `AddGrnPricingAndPoDraft` — hand-added a data backfill (`UPDATE grn_lines SET NetUnitCost = UnitCost, LineTotal = ReceivedValue`) so existing GRN lines stay consistent with their already-posted FIFO layers; `PoUnitPrice` left NULL for historical rows (no retroactive variance). `Down()` drops the six columns cleanly.