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.Core/Entities/SupportingEntities.cs

@@ -45,6 +45,7 @@ public class Vendor : BaseEntity
     public virtual ICollection<BillPayment> BillPayments { get; set; } = new List<BillPayment>();
     public virtual ICollection<Expense> Expenses { get; set; } = new List<Expense>();
     public virtual Account? DefaultExpenseAccount { get; set; }
+    public virtual ICollection<InventoryCategoryLookup> Categories { get; set; } = new List<InventoryCategoryLookup>();
 }
 
 public class InventoryTransaction : BaseEntity