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/ERPCore/Infra/Persistence/Configurations/ItemConfiguration.cs

@@ -19,6 +19,9 @@ public sealed class ItemConfiguration : IEntityTypeConfiguration<Item>
         builder.Property(i => i.Description).HasMaxLength(1000);
         builder.Property(i => i.TaxClass).HasMaxLength(20);
 
+        // Sales-only fixed selling price; nullable (null ⇒ sell at stock/FIFO value).
+        builder.Property(i => i.SalePrice).HasPrecision(18, 4);
+
         builder.Property(i => i.StockNature)
             .HasConversion<string>().HasMaxLength(20).IsRequired();
         builder.Property(i => i.TrackingMode)

Backend/ERPCore/Infra/Persistence/Migrations/ErpDbContextModelSnapshot.cs

@@ -385,6 +385,10 @@ namespace ERPCore.Infra.Persistence.Migrations
                         .HasColumnType("xid")
                         .HasColumnName("xmin");
 
+                    b.Property<decimal?>("SalePrice")
+                        .HasPrecision(18, 4)
+                        .HasColumnType("numeric(18,4)");
+
                     b.Property<string>("Sku")
                         .IsRequired()
                         .HasMaxLength(50)