EKÁER integration, product search, and schema updates

- Added EKÁER (NAV) service registrations and config to PluginNopStartup
- Extended MgProductDto/IMgProductDto with Sku property
- Raised min search term length to 3 in autocomplete endpoints
- Increased ProductSearchAutoComplete maxResults to 300
- Refactored product search to filter by AvailableQuantity > 0
- Closed GTIN/VTSZ issue in EKAER_ISSUES.md, moved to data model topic
- Added SCHEMA.md with Toon domain model, including Currency property for partners

Mango.Nop.Core/Dtos/MgProductDto.cs

@@ -17,6 +17,8 @@ public abstract class MgProductDto : MgEntityBase, /*Product,*/ IMgProductDto//I
     public string ShortDescription { get; set; }
     public string FullDescription { get; set; }
 
+    public string Sku { get; set; }
+
     public int WarehouseId { get; set; }
     public decimal Price { get; set; }
     public int StockQuantity { get; set; }

Mango.Nop.Core/Interfaces/IMgProductDto.cs

@@ -16,6 +16,7 @@ public interface IMgProductDto : IEntityInt, ILocalizedEntity, ISlugSupported, I
     string ShortDescription { get; set; }
     string FullDescription { get; set; }
 
+    string Sku { get; set; }
     int WarehouseId { get; set; }
     decimal Price { get; set; }
     int StockQuantity { get; set; }