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/Views/Jobs/Details.cshtml

@@ -2489,7 +2489,9 @@
     "useMetric": @Json.Serialize((bool)(ViewBag.UseMetric ?? false)),
     "pricingUrl": "@Url.Action("CalculatePricing", "Jobs")",
     "itemsFieldPrefix": "JobItems",
-    "aiRecalcUrl": "@Url.Action("AiRecalcPrice", "Quotes")"
+    "aiRecalcUrl": "@Url.Action("AiRecalcPrice", "Quotes")",
+    "customFormulaTemplates": @Json.Serialize(ViewBag.CustomFormulaTemplates ?? new List<object>()),
+    "formulaEvalUrl": "@Url.Action("EvaluateFormula", "CompanySettings")"
 }
 </script>
 

src/PowderCoating.Web/Views/Jobs/EditItems.cshtml

@@ -179,7 +179,9 @@
     "useMetric": @Json.Serialize((bool)(ViewBag.UseMetric ?? false)),
     "pricingUrl": "@Url.Action("CalculatePricing", "Jobs")",
     "itemsFieldPrefix": "JobItems",
-    "aiRecalcUrl": "@Url.Action("AiRecalcPrice", "Quotes")"
+    "aiRecalcUrl": "@Url.Action("AiRecalcPrice", "Quotes")",
+    "customFormulaTemplates": @Json.Serialize(ViewBag.CustomFormulaTemplates ?? new List<object>()),
+    "formulaEvalUrl": "@Url.Action("EvaluateFormula", "CompanySettings")"
 }
 </script>