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/templates/_widget_dialog_tasks.html

@@ -59,6 +59,8 @@
   </form>
 </section>
 
+{% include "_widget_border_fields.html" %}
+
 <section class="card" style="margin-top: 20px;">
   <h2 class="card-title">Preview</h2>
   <p class="sub">How this widget currently renders.</p>