Fix NCalc case sensitivity and add formula validation

- Normalize IF/Abs/Pow/etc. to lowercase before evaluation so AI-generated
  or manually typed uppercase function names no longer cause "Function not
  found" errors
- Add NormalizeAndValidate() which normalizes then does a parse-only check
  on save — invalid formulas are rejected with a clear error before storing
- Update AI system prompt to list all functions in lowercase and explicitly
  call out case-sensitivity; add if() to the supported function list
- Add collapsible NCalc quick-reference panel in the formula editor showing
  all operators, functions (lowercase), built-in variables, and an example

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

src/PowderCoating.Application/Interfaces/ICustomFormulaAiService.cs

@@ -19,4 +19,12 @@ public interface ICustomFormulaAiService
     /// Safe server-side only — no user-controlled code execution.
     /// </summary>
     EvaluateFormulaResponse EvaluateFormula(EvaluateFormulaRequest request);
+
+    /// <summary>
+    /// Normalizes NCalc built-in function names to lowercase (IF→if, Abs→abs, etc.) then
+    /// attempts a parse-only evaluation to catch syntax errors before the formula is saved.
+    /// Returns the normalized formula string and a null error on success, or the original
+    /// formula and an error message on failure.
+    /// </summary>
+    (string NormalizedFormula, string? Error) NormalizeAndValidate(string formula);
 }