App tree: keep expanded folders open; only the - sign closes one

Rebuilding the tree (live child loads, edits) now restores every row that
was expanded, matched by component id or name path. Double-click no longer
toggles a row through ttk's default binding.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main.py

@@ -1232,18 +1232,23 @@ def _open_special_view(comp):
 
 
 def on_tree_double_click(event):
-    """Double-click on a tree node opens its special view (or Properties)."""
+    """Double-click on a tree node opens its special view (or Properties).
+
+    Returns "break" so ttk's own double-click does not also toggle the row: only the
+    +/- sign opens or closes a node.
+    """
     tree = app_state.tree
     item_id = tree.identify_row(event.y)
     if not item_id:
-        return
+        return "break"
     comp = app_state.tree_element_map.get(item_id)
     if comp is None:
-        return
+        return "break"
     if tree.parent(item_id) == "":
         show_properties(comp)
     else:
         _open_special_view(comp)
+    return "break"
 
 
 def on_canvas_double_click(event):