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/ChoiceButton.qml

@@ -18,8 +18,9 @@ Rectangle {
 
   // Panel-cursor flag. Same role as PillButton.hasCursor: panels that own
   // their own cursor state bind this to drive the keyboard highlight
-  // separately from real activeFocus. Visuals match the activeFocus look
-  // (foreground 2px border) so cursor and Tab focus read the same.
+  // separately from real activeFocus. Cursor renders as a fill only —
+  // CursorSurface is the canonical chrome — while Tab focus adds the
+  // accent border ring on top.
   property bool hasCursor: false
   property bool borderlessHighlight: false
 
@@ -48,7 +49,7 @@ Rectangle {
     ? accent
     : (activeFocus ? foreground : Qt.rgba(foreground.r, foreground.g, foreground.b, 0.4))
   border.width: borderlessHighlight ? (activeFocus ? 2 : 0)
-    : (selected ? 2 : (activeFocus || hasCursor ? 2 : 1))
+    : (selected ? 2 : (activeFocus ? 2 : 1))
 
   Behavior on color { ColorAnimation { duration: 100 } }