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.

server/app/static/widget_dialog_tasks.js

@@ -88,6 +88,7 @@ function initTasksDialog() {
 
   document.getElementById('tasks-preview-refresh').addEventListener('click', loadTasksPreview);
   loadTasksPreview();
+  initBorderFields();
 }
 
 function closeTasksDialog() {