A Widget-level property (border_style/border_thickness/border_color_index),
not a per-type config field, since every widget type can have one -- drawn
once centrally in device.py's _render_widgets before compositing, using
an exact panel palette color so it never dithers. Styles: solid, dashed,
dotted, and a fancy double-line picture-frame-mat look. Configurable from
a shared "Border" card in every widget's gear-icon dialog.
