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

src/myfasthtml/controls/TreeView.py

@@ -184,10 +184,10 @@ class TreeView(MultipleInstance):
     self._state = TreeViewState(self)
     self.conf = conf or TreeViewConf()
     self.commands = Commands(self)
-
+    
     if items:
       self._state.items = items
-
+    
     if self.conf.icons:
       self._state.icon_config = self.conf.icons
   
@@ -350,6 +350,7 @@ class TreeView(MultipleInstance):
   
   def _save_rename(self, node_id: str, node_label: str):
     """Save renamed node with new label."""
+    logger.debug(f"_save_rename {node_id=}, {node_label=}")
     if node_id not in self._state.items:
       raise ValueError(f"Node {node_id} does not exist")
     
@@ -393,7 +394,9 @@ class TreeView(MultipleInstance):
     """Select a node."""
     if node_id not in self._state.items:
       raise ValueError(f"Node {node_id} does not exist")
-    
+
+    # Cancel edit mode when selecting
+    self._state.editing = None
     self._state.selected = node_id
     return self
   
@@ -401,11 +404,11 @@ class TreeView(MultipleInstance):
     """Render action buttons for a node (visible on hover)."""
     is_leaf = len(self._state.items[node_id].children) == 0
     conf = self.conf
-
-    add_visible    = conf.add_leaf    if is_leaf else conf.add_node
-    edit_visible   = conf.edit_leaf   if is_leaf else conf.edit_node
+    
+    add_visible = conf.add_leaf if is_leaf else conf.add_node
+    edit_visible = conf.edit_leaf if is_leaf else conf.edit_node
     delete_visible = conf.delete_leaf if is_leaf else conf.delete_node
-
+    
     buttons = []
     if add_visible:
       buttons.append(mk.icon(add_circle20_regular, command=self.commands.add_child(node_id)))
@@ -413,7 +416,7 @@ class TreeView(MultipleInstance):
       buttons.append(mk.icon(edit20_regular, command=self.commands.start_rename(node_id)))
     if delete_visible:
       buttons.append(mk.icon(delete20_regular, command=self.commands.delete_node(node_id)))
-
+    
     return Div(*buttons, cls="mf-treenode-actions")
   
   def _render_node(self, node_id: str, level: int = 0):