Add Custom Formula Item Templates with AI generation and wizard integration

Introduces per-company reusable NCalc2 pricing formula templates for complex
fabricated items (roof curbs, enclosures, welded frames). Templates support
two output modes — FixedRate (formula yields a dollar amount) and SurfaceAreaSqFt
(formula yields sq ft fed into the standard coating engine). Includes:

- CustomItemTemplate entity, migration (AddCustomItemTemplates), IUnitOfWork repo
- IsCustomFormulaItem / CustomItemTemplateId / FormulaFieldValuesJson flags on
  QuoteItem, JobItem, CreateQuoteItemDto; mapped in all 3 JobItemAssemblyService
  overloads and all existingItemsData JSON projections + pageMeta blocks
- ICustomFormulaAiService / CustomFormulaAiService: Claude-powered formula
  generator (natural language + optional diagram image) and NCalc2 evaluator
- CompanySettings CRUD endpoints: GetCustomItemTemplates, Create/Update/Delete,
  UploadTemplateDiagram, TemplateDiagram (blob serve), EvaluateFormula, GenerateFormulaFromAi
- Company Settings "Custom Formulas" tab + cfModal + company-settings-custom-formulas.js
- item-wizard.js: formula item type card, renderFormulaFields, wzFormulaRecalc
  (live evaluate via POST), collectStep2 formula branch, buildCardHtml / emitHiddenFields
- Formula badge in Quotes/Details and Jobs/Details; AI badge gap fixed in Jobs/Details
- Help article (CustomFormulaTemplates.cshtml), Help Index card, HelpController action,
  HelpKnowledgeBase entry; 225/225 unit tests passing

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

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

@@ -384,6 +384,9 @@
         isGenericItem        = item.IsGenericItem,
         isLaborItem          = item.IsLaborItem,
         isAiItem             = item.IsAiItem,
+        isCustomFormulaItem  = item.IsCustomFormulaItem,
+        customItemTemplateId = item.CustomItemTemplateId,
+        formulaFieldValuesJson = item.FormulaFieldValuesJson,
         requiresSandblasting = item.RequiresSandblasting,
         requiresMasking      = item.RequiresMasking,
         notes                = item.Notes,
@@ -425,6 +428,8 @@
     "aiUploadUrl": "@Url.Action("UploadAiPhoto", "Quotes")",
     "aiAnalyzeUrl": "@Url.Action("AiAnalyzeItem", "Quotes")",
     "aiPhotoQuotesEnabled": @Json.Serialize((bool)(ViewBag.AiPhotoQuotesEnabled ?? true)),
+    "customFormulaTemplates": @Json.Serialize(ViewBag.CustomFormulaTemplates ?? new List<object>()),
+    "formulaEvalUrl": "@Url.Action("EvaluateFormula", "CompanySettings")",
     "itemsFieldPrefix": "JobItems",
     "aiRecalcUrl": "@Url.Action("AiRecalcPrice", "Quotes")"
 }