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.