PR summary

The RadioButtons widget now supports arranging buttons in a 2D grid by passing a list of lists of strings as the labels parameter. Each inner list represents a row in the grid.

Key features:

• Active index and index_selected remain as single integers for the
  flattened array (reading left-to-right, top-to-bottom)
• Column positions are automatically calculated based on the maximum
  text width in each column for optimal spacing
• Text offset is now consistent between 1D and 2D layouts (0.10)
• Includes new example: galleries/examples/widgets/radio_buttons_grid.py

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]

PR checklist

• closes Horizontal radio buttons #13374
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines