paint drag handle background to match adjacent panel

The non-line pixels in the 5px drag handle now fill with the
adjacent panel's background color (sidebar or context) instead
of being transparent. The 1px divider line stays on the inner
edge touching the center column.

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

lib/kernel/src/panels/drag_resize.dart

@@ -46,9 +46,10 @@ class _DragResizeHandleState extends State<DragResizeHandle> {
         onPointerDown: _onDown,
         onPointerMove: _onMove,
         onPointerUp: _onUp,
-        child: SizedBox(
+        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,
           child: Align(
             alignment: widget.slot == Slots.sidebar ? Alignment.centerRight : Alignment.centerLeft,
             child: Container(