Refactor: extract shared helpers, fix field drift, add assembly services

- IJobItemAssemblyService / IQuotePricingAssemblyService: centralize job item
  and quote pricing construction that was duplicated across create, rework copy,
  and quote-to-job conversion paths
- BlobFileHelper: single ValidateUpload/GetContentType/SanitizeFileName used by
  6 blob services (JobPhoto, QuotePhoto, ProfilePhoto, CompanyLogo, Equipment,
  Catalog) and BillsController + ExpensesController, removing 8 private copies
- PagedResult<T>.From(): static factory eliminates 6-line boilerplate in 11
  controllers (Appointments, Customers, Equipment, Inventory, Invoices, Jobs,
  Maintenance, CompanyUsers, PlatformUsers, Quotes, Vendors)
- AccountingDropdownHelper: single LoadAsync() call replaces duplicate
  vendor/account/job queries in BillsController and ExpensesController
- JobTemplateItem: add IsSalesItem + Sku fields with migration; propagate
  through JobTemplatesController snapshot copy and GetTemplatesJson projection,
  and JobsController template-application path
- Test assertions updated for standardized BlobFileHelper error messages

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

src/PowderCoating.Application/DTOs/Common/PagedResult.cs

@@ -3,6 +3,22 @@ namespace PowderCoating.Application.DTOs.Common;
 public class PagedResult<T>
 {
     public IEnumerable<T> Items { get; set; } = new List<T>();
+
+    /// <summary>
+    /// Creates a PagedResult populated from a GridRequest, avoiding repetitive property
+    /// assignments across every Index action. SortColumn, SortDirection, and SearchTerm
+    /// are copied from the grid so the model carries full state for view binding.
+    /// </summary>
+    public static PagedResult<T> From(GridRequest grid, IEnumerable<T> items, int totalCount) => new()
+    {
+        Items = items,
+        PageNumber = grid.PageNumber,
+        PageSize = grid.PageSize,
+        TotalCount = totalCount,
+        SortColumn = grid.SortColumn,
+        SortDirection = grid.SortDirection,
+        SearchTerm = grid.SearchTerm
+    };
     public int PageNumber { get; set; }
     public int PageSize { get; set; }
     public int TotalCount { get; set; }