Add formula template walkthrough and UX improvements

- 7-step guided walkthrough modal (concept, output modes, fields, formula,
  testing, box example, cylinder example); auto-shown first time the tab
  opens with no templates; always accessible via "How it works" button
- Variable pill badges below formula input showing all valid field names + rate;
  update live as fields are added/renamed; clickable to insert at cursor
- Fix: Add Field no longer shows validation error on blank new rows; validation
  only fires once the user has typed something
- Help article: added Common Formula Patterns section with box, cylinder, and
  flat panel worked examples (fields table + formula + expected output)
- HelpKnowledgeBase updated with pattern examples and walkthrough note

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

src/PowderCoating.Web/Helpers/HelpKnowledgeBase.cs

@@ -1401,7 +1401,11 @@ public static class HelpKnowledgeBase
         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
+        Common formula patterns (all Fixed Rate, divide inches by 144 to get sqft):
+        - 6-sided box: fields l_in/w_in/h_in → 2*(l_in*w_in + l_in*h_in + w_in*h_in) / 144 * rate
+        - Cylinder: fields d_in/h_in → (3.14159 * d_in * h_in + 2 * 3.14159 * Pow(d_in/2, 2)) / 144 * rate
+        - Flat panel: fields l_in/w_in → l_in * w_in / 144 * rate
+        Walkthrough: first time opening Custom Formulas tab with no templates triggers a 7-step guided tour automatically; also accessible via "How it works" button
         Help article: Help → Custom Formula Item Templates
         """;