Fix custom formula wizard bugs and add field name validation

- Fix Add Field blanking inputs: cfFields was IIFE-scoped so inline oninput
  handlers couldn't reach it; expose cfUpdateField on window
- Fix ManualUnitPrice dropped in buildItemFromData: condition excluded
  isCustomFormulaItem, causing FixedRate items to reprice from scratch
- Fix formula card missing on job pages: load CustomFormulaTemplates in
  PopulateJobItemDropDownsAsync so Details, EditItems, and Edit all get it;
  add customFormulaTemplates + formulaEvalUrl to Details and EditItems pageMeta
- Add NCalc field name validation: client-side inline feedback (is-invalid +
  message on oninput) and pre-save sweep; server-side ValidateTemplateFields
  on Create and Update; rules: letter-start, letters/digits/underscores only,
  no duplicates, "rate" reserved

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

src/PowderCoating.Web/Controllers/JobsController.cs

@@ -3340,6 +3340,13 @@ public class JobsController : Controller
         var useMetric = await _tenantContext.UseMetricSystemAsync();
         ViewBag.UseMetric = useMetric;
         ViewBag.AreaUnit = _measurementService.GetAreaUnitLabel(useMetric);
+
+        var formulaTemplates = await _unitOfWork.CustomItemTemplates.FindAsync(t => t.CompanyId == companyId && t.IsActive);
+        ViewBag.CustomFormulaTemplates = formulaTemplates.OrderBy(t => t.DisplayOrder).ThenBy(t => t.Name)
+            .Select(t => new { id = t.Id, name = t.Name, description = t.Description, outputMode = t.OutputMode,
+                fieldsJson = t.FieldsJson, formula = t.Formula, defaultRate = t.DefaultRate, rateLabel = t.RateLabel,
+                diagramImagePath = string.IsNullOrEmpty(t.DiagramImagePath) ? null
+                    : Url.Action("TemplateDiagram", "CompanySettings", new { templateId = t.Id }) }).ToList();
     }
 
     /// <summary>