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/kernel/src/panels/drag_resize.dart

@@ -49,7 +49,7 @@ class _DragResizeHandleState extends State<DragResizeHandle> {
         child: Container(
           width: widget.axis == Axis.horizontal ? widget.thickness : null,
           height: widget.axis == Axis.vertical ? widget.thickness : null,
-          color: widget.slot == Slots.sidebar ? tokens.sidebarBackground : tokens.panelBackground,
+          color: tokens.chromeBackground,
           child: Align(
             alignment: widget.slot == Slots.sidebar ? Alignment.centerRight : Alignment.centerLeft,
             child: Container(