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/wwwroot/js/item-wizard.js

@@ -2865,7 +2865,7 @@ function buildItemFromWizard() {
         surfaceAreaSqFt:   d.surfaceAreaSqFt || 0,
         estimatedMinutes:  d.estimatedMinutes || 0,
         catalogItemId:     d.catalogItemId || null,
-        manualUnitPrice:   isAi ? (d.manualUnitPrice ?? null) : (d.isGenericItem || d.isSalesItem ? (d.manualUnitPrice ?? null) : null),
+        manualUnitPrice:   isAi ? (d.manualUnitPrice ?? null) : (d.isGenericItem || d.isSalesItem || d.isCustomFormulaItem ? (d.manualUnitPrice ?? null) : null),
         powderCostOverride: d.powderCostOverride ?? null,
         isGenericItem:     !!d.isGenericItem,
         isLaborItem:       !!d.isLaborItem,