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/Helpers/HelpKnowledgeBase.cs

@@ -1388,5 +1388,21 @@ public static class HelpKnowledgeBase
         ---
 
         Remember: if the user asks something outside this knowledge base or asks for something very specific to their data, acknowledge the limits and point them to the relevant page or the Help Center.
+
+        ---
+
+        **Custom Formula Item Templates (Company Settings → Custom Formulas):**
+        Reusable NCalc pricing formulas for complex fabricated items (roof curbs, electrical enclosures, welded frames). Each template has a list of measurement fields and a formula expression. Two output modes:
+        - Fixed Rate: formula produces a dollar amount → stored as ManualUnitPrice × Qty
+        - Surface Area: formula produces sq ft → standard coating engine prices it
+
+        Creating a template: New Template → enter name + output mode + fields (name, label, unit, default value) → write NCalc formula using field names → Run to test → optionally upload a diagram image → Save
+        AI Generator: enter description in "AI Formula Generator" box in the template editor → Claude suggests formula + fields + mode → review and save
+        Using in wizard: item wizard shows "Custom Formula Item" card if active templates exist → choose template → template diagram shown for reference → enter measurements → Calculate → verify result → continue to coatings/prep steps
+        Formula variable names: snake_case, letters/digits/underscores only. Reserved variable: "rate" (pre-populated from Default Rate).
+        NCalc syntax: +, -, *, /, %, Pow(b,e), Abs(x), Round(x,d), Max(a,b), Min(a,b), Sqrt(x)
+        Example formula (box, inches): 2*(l*w + l*h + w*h) / 144 * rate
+        Help article: Help → Custom Formula Item Templates
         """;
+
 }