Make Button state composition visible in ButtonGroup and the gallery

Three small fixes around the new Button + ButtonGroup so the cursor is
always visible and the section reads as a single Button showing flag
combinations.

ButtonGroup now exposes cursorIndex + hovered(index, isHovered) so
panels can drive a single Button's hasCursor without reaching past
ButtonGroup's API. Every chip gets bordered: true so the row reads as
a real picker — distinct outlines, not just a tinted fill that vanishes
into the background.

Button's border treatment grows two paths so the cursor is always
visible regardless of other state: bordered + hot recolors the border
to the accent and thickens to the focus-ring width, and selected + hot
thickens the existing accent border to the focus-ring width (so the
cursor landing on the chosen option still reads as cursor presence,
instead of disappearing into the selected fill).

Gallery's Button section pairs each demo button with a small caption
('idle', 'active', 'icon only', 'icon + active', 'bordered +
focusable') so the row reads as one Button with its state flags side
by side rather than five unrelated buttons. The ButtonGroup section
gets the cursor wiring (cursorIndex + onHovered) plus an
onFocusedChanged auto-scroll so the cursor visual is reachable via
j/k h/l from the panel cursor model.

shell/Ui/Button.qml

@@ -72,13 +72,23 @@ Rectangle {
     : active               ? Style.selectedFill
     : background
 
+  // Border color follows the same precedence as fill: focus ring wins,
+  // then selected, then cursor on bordered (paints accent so the chip
+  // structure clearly reads as "cursor is here"), then plain bordered
+  // (foreground), then nothing.
   border.color: _showFocusRing ? Style.focusBorderColor
     : selected                 ? accent
+    : (bordered && hot)        ? Style.focusBorderColor
     : bordered                 ? foreground
     : Style.idleBorderColor
 
+  // selected+hot thickens to the focus-ring width so the cursor remains
+  // visible on the chosen option (otherwise selected's accent fill+border
+  // masks any hot fill). bordered+hot also thickens so the chip cursor
+  // reads as a deliberate state change rather than a faint tint.
   border.width: _showFocusRing ? Style.focusBorderWidth
-    : selected                 ? Math.max(Style.borderWidth, 2)
+    : selected                 ? (hot ? Style.focusBorderWidth : Math.max(Style.borderWidth, 2))
+    : (bordered && hot)        ? Style.focusBorderWidth
     : bordered                 ? Style.borderWidth
     : 0