theme_gtsummary_roche() leaves default left/right cell padding

`theme_gtsummary_roche()` zeros top and bottom cell padding for flextable output but leaves flextable's default 5pt left/right padding, adding ~0.19" of unused horizontal space to every column. Gridline-free clinical listings don't need it and it wastes width on dense landscape tables.

Start at the theme_gtsummary_roche() entry point and inspect how it configures flextable output. The work is done when the theme removes the default left and right cell padding as well as the existing top and bottom padding, without leaving the extra horizontal space in clinical listings.

Written by the indexing model from the issue text.