feat: implement new grid creation with inline rename in DataGridsManager

- Add new_grid() method to create empty DataGrid under selected folder/leaf parent
  - Generate unique sheet names (Sheet1, Sheet2, ...) with _generate_unique_sheet_name()
  - Auto-select and open new node in edit mode for immediate renaming
  - Fix TreeView to cancel edit mode when selecting any node
  - Wire "New grid" icon to new_grid() instead of clear_tree()
  - Add 14 unit tests covering new_grid() scenarios and TreeView behavior

tests/controls/test_treeview.py

@@ -583,6 +583,44 @@ class TestTreeviewBehaviour:
     assert len(tree_view._state.items) == 1, "Node should not have been added to items"
     assert tree_view._state.items[node1.id] == node2, "Node should not have been replaced"
 
+  def test_selecting_node_cancels_edit_mode(self, root_instance):
+    """Test that selecting a node cancels any active edit mode."""
+    tree_view = TreeView(root_instance)
+    node1 = TreeNode(label="Node 1", type="folder")
+    node2 = TreeNode(label="Node 2", type="folder")
+
+    tree_view.add_node(node1)
+    tree_view.add_node(node2)
+
+    # Start editing node1
+    tree_view._start_rename(node1.id)
+    assert tree_view._state.editing == node1.id
+
+    # Select node2
+    tree_view._select_node(node2.id)
+
+    # Edit mode should be cancelled
+    assert tree_view._state.editing is None
+    assert tree_view._state.selected == node2.id
+
+  def test_selecting_same_editing_node_cancels_edit_mode(self, root_instance):
+    """Test that selecting the same node being edited cancels edit mode."""
+    tree_view = TreeView(root_instance)
+    node = TreeNode(label="Node", type="folder")
+
+    tree_view.add_node(node)
+
+    # Start editing the node
+    tree_view._start_rename(node.id)
+    assert tree_view._state.editing == node.id
+
+    # Select the same node
+    tree_view._select_node(node.id)
+
+    # Edit mode should be cancelled
+    assert tree_view._state.editing is None
+    assert tree_view._state.selected == node.id
+
 
 class TestTreeViewRender:
   """Tests for TreeView HTML rendering."""