Standardize cursor chrome to fill-only across qs.Ui

CursorSurface defines the canonical cursor chrome: a tinted fill, no
border. Three components had inconsistent treatments that the recent
standardization pass left half-finished:

- PillButton kept a 1px foreground border on hot (cursor or hover)
  via a cursorBordered: true default.
- CursorPill specifically had to override cursorBordered: false to
  match the rest of the kit.
- ChoiceButton thickened its border to 2px on hasCursor, which mixed
  the cursor visual with the Tab-focus visual.

Drop the cursorBordered escape hatch from PillButton (no caller other
than CursorPill ever used it), drop CursorPill's now-redundant
override, and remove hasCursor from ChoiceButton's border.width
thickening. Update Toggle's class comment (it still claimed cursor
mirrored Tab focus, which f3ba5055 broke without updating the doc).

Now: hot/hasCursor = fill only, activeFocus = accent border ring,
selected/active = stronger fill or accent border per component
semantic. One canonical pattern across the kit.

shell/Ui/CursorPill.qml

@@ -20,8 +20,6 @@ import QtQuick
 PillButton {
   id: root
 
-  cursorBordered: false
-
   signal hovered(bool isHovered)
 
   HoverHandler {