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/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>
 }