Add vendor supply categories with inventory auto-filter

Vendors can now be tagged with one or more inventory categories (Powder,
Chemical, etc.) via checkboxes on the Create/Edit form. The inventory
Create/Edit vendor dropdown automatically filters to matching vendors when
a category is selected; falls back to all vendors if none are tagged.
Includes migration AddVendorCategories (VendorInventoryCategories join table).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Application/DTOs/Vendor/VendorDtos.cs

@@ -125,6 +125,8 @@ public class CreateVendorDto
 
     [Display(Name = "Default Expense Account")]
     public int? DefaultExpenseAccountId { get; set; }
+
+    public List<int> CategoryIds { get; set; } = new();
 }
 
 // ============================================================================
@@ -209,4 +211,6 @@ public class UpdateVendorDto
 
     [Display(Name = "Default Expense Account")]
     public int? DefaultExpenseAccountId { get; set; }
+
+    public List<int> CategoryIds { get; set; } = new();
 }