Add `render_layout_prompt` and `parse_layout_input` to ui.rs so that
the first-run wizard asks the user to choose a tmux layout (1–5 or by
name) for each window that has 2 or more panes. Previously, layout was
always silently set to None.
Also update `render_config_created` to display the chosen layout in the
post-setup summary.
Closes: layout never being set during interactive setup