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.

Frontend/erp-system/lib/validations/master-data.ts

@@ -73,3 +73,23 @@ export function validateVariantItemForm(input: {
   if (!input.hasVariants) errors.variants = "Check at least one variant category and add its values"
   return errors
 }
+
+/**
+ * Fixed-price mode requires every generated variant to carry a sale price > 0
+ * (docs/20 §3.1). Returns a map keyed by variant key → message; empty when valid.
+ * In "stock" mode there is nothing to validate (prices are sent as null).
+ */
+export function validateVariantPrices(
+  variantKeys: string[],
+  priceFor: (key: string) => string,
+): Record<string, string> {
+  const errors: Record<string, string> = {}
+  for (const key of variantKeys) {
+    const raw = priceFor(key).trim()
+    const value = Number(raw)
+    if (raw === "" || Number.isNaN(value) || value <= 0) {
+      errors[key] = "Enter a price greater than 0"
+    }
+  }
+  return errors
+}