fix(ui): set Description in phase-level progress reports

ReportProgress was not setting Description, so the Detail row always
showed the default em-dash. Now each phase report includes a meaningful
description, and UpdateProgress always updates the label (resetting to
em-dash when null).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Forms/NestProgressForm.cs

@@ -38,8 +38,9 @@ namespace OpenNest.Forms
             densityValue.Text = progress.BestDensity.ToString("P1");
             remnantValue.Text = $"{progress.UsableRemnantArea:F1} sq in";
 
-            if (!string.IsNullOrEmpty(progress.Description))
-                descriptionValue.Text = progress.Description;
+            descriptionValue.Text = !string.IsNullOrEmpty(progress.Description)
+                ? progress.Description
+                : "\u2014";
         }
 
         public void ShowCompleted()