Unify PillButton, CursorPill, ChoiceButton into qs.Ui.Button + ButtonGroup

The three components were three takes on the same shape \u2014 a clickable
rectangle with text/icon, a hot/hover state, an optional persistent
border, and an optional 'selected' or 'active' highlight. CursorPill
was a 30-line PillButton wrapper that added a hovered() signal;
ChoiceButton was effectively PillButton with selected: bool painting
an accent fill+border.

Collapse them into a single qs.Ui.Button. State flags compose
independently:

  hasCursor / hover         hot fill
  active                    persistent foreground-tint fill
  selected                  accent fill + accent border
  bordered: true            persistent 1px idle border (form primaries)
  focusable: true           Tab focus paints the accent ring
  pressed                   pressed fill

The hovered(bool) signal is now built-in, so CursorPill's wrapper is
unnecessary. ButtonGroup wraps a Row+Repeater for the form-style
'pick one of N' pattern; panel-cursor-driven cases still compose
Buttons directly in a Row with per-instance hasCursor wiring.

Theme tokens move into a new [style] section in shell.toml:

  border-width        = 1
  focus-border-width  = 3
  idle-border-alpha   = 0.4
  hot-fill-alpha      = 0.08
  selected-fill-alpha = 0.18
  pressed-fill-alpha  = 0.22
  focus-fill-alpha    = 0.22

Style.qml parses these out of the same shell.toml [font] / [bar]
already reads, and exposes pre-computed Style.hotFill / selectedFill /
pressedFill / idleBorderColor / selectedAccentFill / borderWidth +
the existing focusBorder* tokens. Themes that don't ship a [style]
section get the previous defaults unchanged.

Dev gallery consolidates three sections (PillButton, CursorPill,
ChoiceButton) into Button + ButtonGroup, with the cursor model
sections renamed accordingly.

default/themed/shell.toml.tpl

@@ -12,6 +12,17 @@ active     = "{{ color1 }}"
 size-horizontal = 26
 size-vertical   = 28
 
+[style]
+# State alphas used by every interactive surface in the kit (Button,
+# Toggle, TextField, etc.). Foreground-tinted unless noted.
+border-width        = 1     # idle 1px border on inputs and bordered buttons
+focus-border-width  = 3     # accent ring on Tab focus
+idle-border-alpha   = 0.4   # alpha for the idle 1px foreground border
+hot-fill-alpha      = 0.08  # cursor / hover fill
+selected-fill-alpha = 0.18  # selected / active / current fill
+pressed-fill-alpha  = 0.22  # button pressed
+focus-fill-alpha    = 0.22  # accent fill behind Tab focus ring
+
 [font]
 # base-size is the rem root for the type scale. Every Style.font.<token>
 # derives from it (e.g. body = base, subtitle ≈ base * 1.083,