make hat bar full-width, separators 1px visible

Hat bar is now a single continuous row above the three columns
instead of per-column segments. Left: macOS traffic lights or
empty. Center: project > branch. Right: minimize/maximize/close
on Linux or empty. Column separators keep 4px hit area but show
a 1px centered line.

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

lib/kernel/src/panels/drag_resize.dart

@@ -44,10 +44,16 @@ class _DragResizeHandleState extends State<DragResizeHandle> {
         onPointerDown: _onDown,
         onPointerMove: _onMove,
         onPointerUp: _onUp,
-        child: Container(
+        child: SizedBox(
           width: widget.axis == Axis.horizontal ? widget.thickness : null,
           height: widget.axis == Axis.vertical ? widget.thickness : null,
-          color: color,
+          child: Center(
+            child: Container(
+              width: widget.axis == Axis.horizontal ? 1 : null,
+              height: widget.axis == Axis.vertical ? 1 : null,
+              color: color,
+            ),
+          ),
         ),
       ),
     );