add chrome token set for frame surfaces

New tokens: chromeBackground, chromeForeground, chromeBorder —
shared root for hat bar, sidebar, context panel, status bar,
spines, and drag handles. All resolve to bgSunken/textDim/border
in the palette by default. Themes can override individually to
diverge surfaces. Replaces direct sidebarBackground/statusBar
references in chrome contexts. Updated theme-ui skill.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/widgets/src/clide_spine.dart

@@ -45,7 +45,7 @@ class _ClideSpineState extends State<ClideSpine> {
           child: Container(
             width: ClideSpine.width,
             decoration: BoxDecoration(
-              color: _hovered ? tokens.sidebarItemHover : tokens.sidebarBackground,
+              color: _hovered ? tokens.sidebarItemHover : tokens.chromeBackground,
               border: Border(
                 left: widget.side == SpineSide.right ? borderSide : BorderSide.none,
                 right: widget.side == SpineSide.left ? borderSide : BorderSide.none,