fix: use Gui.Document.Modified instead of App.Document.Modified (#13)

App.Document has no 'Modified' attribute — it only exists on
Gui.Document. This caused every Silo save to fail with:
  Silo save failed: 'App.Document' object has no attribute 'Modified'

The save itself succeeded but the modified flag was never cleared,
so the document always appeared unsaved.

freecad/silo_origin.py

@@ -392,8 +392,10 @@ class SiloOrigin:
                 obj.SiloPartNumber, str(file_path), properties, comment=""
             )
 
-            # Clear modified flag
-            doc.Modified = False
+            # Clear modified flag (Modified is on Gui.Document, not App.Document)
+            gui_doc = FreeCADGui.getDocument(doc.Name)
+            if gui_doc:
+                gui_doc.Modified = False
 
             return True
         except Exception as e: