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.Application/Services/PricingCalculationService.cs

@@ -288,6 +288,24 @@ public class PricingCalculationService : IPricingCalculationService
             };
         }
 
+        // Custom formula items (FixedRate mode): the wizard evaluated the NCalc formula server-side
+        // and stored the result as ManualUnitPrice. Use it directly — no coating math.
+        // SurfaceAreaSqFt mode: ManualUnitPrice is null; the formula produced sqft which was stored
+        // in SurfaceAreaSqFt, so the item falls through to the standard calculated path below.
+        if (item.IsCustomFormulaItem && item.ManualUnitPrice.HasValue)
+        {
+            var total = item.ManualUnitPrice.Value * item.Quantity;
+            return new QuoteItemPricingResult
+            {
+                MaterialCost  = 0,
+                LaborCost     = 0,
+                EquipmentCost = 0,
+                ItemSubtotal  = total,
+                UnitPrice     = item.ManualUnitPrice.Value,
+                TotalPrice    = total
+            };
+        }
+
         // Sales items (off-the-shelf merchandise) — manual unit price, no coating calculation.
         if (item.IsSalesItem && item.ManualUnitPrice.HasValue)
         {