Add per-widget border option (style, thickness, palette color)

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.

CLAUDE.md

@@ -9,7 +9,6 @@ photos/calendar/whiteboard/weather widget system to the device.
 CURRENT TODO
 -add more actions for buttons (i.e. change widget/layout)
 -Fix spurious button assignment stuff (probably but buttons on widget config with sane defaults)
--widget border option
 -battery life widget
 -sharing layouts with linked users
 -a "coming up this week" widget