fix(blur): the layer is draggable, so adding one drops a rectangle with a resize handle

Source.IsDraggable listed Image/Web/Chat/Alert only, so a blur had no selection box, no corner
handle, and no hit-test — the rectangle existed in the render but could not be grabbed. Adding
Blur to that list gives it the same selection chrome and resize path every placeable layer has
(drag to move, corner dot to resize, one undo snapshot per gesture).

It is selected on add (AddSource already does that), so the rectangle appears already outlined
and ready to size.

Models/Source.cs

@@ -84,7 +84,11 @@ public class Source : SceneElement
         }
     }
 
-    public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox or SourceType.AlertBox;
+    /// <summary>Blur is draggable like a layer: it has no pixels of its own, but the creator places
+    /// and sizes it in the preview, so it needs the same selection box, grab and resize handle
+    /// (creator ruling 2026-11-01).</summary>
+    public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox
+        or SourceType.AlertBox or SourceType.Blur;
 
     /// <summary>Live captures, chat, web sources and media are dynamic; static art is fixed.</summary>
     public override ElementKind Kind => Type is SourceType.DisplayCapture or SourceType.WindowCapture