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())
             {

src/PowderCoating.Web/Views/Inventory/Label.cshtml

@@ -95,6 +95,16 @@
             color: #333;
         }
 
+        .label-location {
+            font-size: 11px;
+            font-weight: 700;
+            color: #333;
+            background: #f0f0f0;
+            border-radius: 4px;
+            padding: 2px 8px;
+            letter-spacing: .03em;
+        }
+
         .label-scan-hint {
             font-size: 9px;
             color: #888;
@@ -158,6 +168,11 @@
             <div class="label-sku" style="color:#777">@Model.Manufacturer</div>
         }
 
+        @if (!string.IsNullOrEmpty(Model.Location))
+        {
+            <div class="label-location">&#128205; @Model.Location</div>
+        }
+
         <div class="label-scan-hint">
             Scan to log usage &bull; Powder Coating Logix
         </div>

src/PowderCoating.Web/Views/Shared/_Pagination.cshtml

@@ -83,4 +83,12 @@
             </nav>
         </div>
     </div>
+    <script>
+        function changePageSize(size) {
+            var url = new URL(window.location.href);
+            url.searchParams.set('pageSize', size);
+            url.searchParams.set('pageNumber', '1');
+            window.location.href = url.toString();
+        }
+    </script>
 }