Fix formula export/import: embed fields as real JSON array, not escaped string

Previously FieldsJson was serialized as an escaped string in the export
file, which was fragile and unreadable. Now parsed into a JsonElement and
embedded as a proper JSON array under the key "fields". Import reads it
back with GetRawText() to reconstruct the stored string. This prevents
the null/empty fields bug caused by manually-edited or round-tripped files.

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

src/PowderCoating.Web/Controllers/CompanySettingsController.cs

@@ -3051,6 +3051,15 @@ public class CompanySettingsController : Controller
         var companyId = _tenantContext.GetCurrentCompanyId()!.Value;
         var templates = await _unitOfWork.CustomItemTemplates.FindAsync(t => t.CompanyId == companyId);
 
+        // Parse FieldsJson into a real JsonElement so it is embedded as a proper JSON array
+        // in the export file rather than as an escaped string. This makes the file human-readable
+        // and avoids round-trip corruption when files are manually edited.
+        static System.Text.Json.JsonElement ParseFields(string? raw)
+        {
+            try { return System.Text.Json.JsonDocument.Parse(raw ?? "[]").RootElement.Clone(); }
+            catch { return System.Text.Json.JsonDocument.Parse("[]").RootElement.Clone(); }
+        }
+
         var export = new
         {
             exportedAt = DateTime.UtcNow,
@@ -3062,7 +3071,7 @@ public class CompanySettingsController : Controller
                     t.Name,
                     t.Description,
                     t.OutputMode,
-                    t.FieldsJson,
+                    Fields = ParseFields(t.FieldsJson),
                     t.Formula,
                     t.DefaultRate,
                     t.RateLabel,
@@ -3142,7 +3151,8 @@ public class CompanySettingsController : Controller
                     Name = name,
                     Description = item.TryGetProperty("description", out var d) ? d.GetString() : null,
                     OutputMode = item.TryGetProperty("outputMode", out var om) ? om.GetString() ?? "FixedRate" : "FixedRate",
-                    FieldsJson = item.TryGetProperty("fieldsJson", out var fj) ? fj.GetString() ?? "[]" : "[]",
+                    // "fields" is a real JSON array in the export; GetRawText() reconstructs the string
+                    FieldsJson = item.TryGetProperty("fields", out var fj) ? fj.GetRawText() : "[]",
                     Formula = item.TryGetProperty("formula", out var f) ? f.GetString() ?? "" : "",
                     DefaultRate = item.TryGetProperty("defaultRate", out var dr) && dr.ValueKind == System.Text.Json.JsonValueKind.Number ? dr.GetDecimal() : null,
                     RateLabel = item.TryGetProperty("rateLabel", out var rl) ? rl.GetString() : null,