Fix three bugs: vendor duplicate check, page size dropdown, label location

- Vendor Create: reject duplicate company names (case-insensitive) before
  saving; works for both the standalone form and the inline quick-add modal
- _Pagination: define changePageSize() JS function (was called but never
  existed, breaking page size dropdown on every paginated list)
- Inventory Label: show bin/location on printed QR code labels

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

src/PowderCoating.Web/Controllers/VendorsController.cs

@@ -215,8 +215,23 @@ public class VendorsController : Controller
         try
         {
             var currentUser = await _userManager.GetUserAsync(User);
+            var companyId = currentUser!.CompanyId;
+
+            var duplicate = await _unitOfWork.Vendors.FirstOrDefaultAsync(
+                v => v.CompanyId == companyId && v.CompanyName.ToLower() == dto.CompanyName.ToLower());
+            if (duplicate != null)
+            {
+                var msg = $"A vendor named '{dto.CompanyName}' already exists.";
+                if (inline)
+                    return Json(new { success = false, errors = new[] { msg } });
+                ModelState.AddModelError(nameof(dto.CompanyName), msg);
+                await PopulateExpenseAccountsAsync();
+                await PopulateVendorCategoriesAsync(dto.CategoryIds);
+                return View(dto);
+            }
+
             var vendor = _mapper.Map<Vendor>(dto);
-            vendor.CompanyId = currentUser!.CompanyId;
+            vendor.CompanyId = companyId;
 
             if (dto.CategoryIds.Any())
             {