align divider line to inner edge, add side panel padding

The 1px visible line in the 5px drag handle now sits on the
innermost pixel (touching the center column) instead of centered.
Side panels (sidebar and context) get 2px padding on all sides
except the bottom, creating a balanced inset that matches the
divider's visual weight.

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

lib/app.dart

@@ -441,6 +441,7 @@ class _SidebarSlot extends StatelessWidget {
     return Container(
       color: tokens.sidebarBackground,
       alignment: Alignment.topLeft,
+      padding: const EdgeInsets.fromLTRB(2, 2, 2, 0),
       child: active.build(context),
     );
   }
@@ -557,6 +558,7 @@ class _ContextSlot extends StatelessWidget {
     return Container(
       color: tokens.panelBackground,
       alignment: Alignment.topLeft,
+      padding: const EdgeInsets.fromLTRB(2, 2, 2, 0),
       child: active.build(context),
     );
   }

lib/kernel/src/panels/drag_resize.dart

@@ -49,7 +49,8 @@ class _DragResizeHandleState extends State<DragResizeHandle> {
         child: SizedBox(
           width: widget.axis == Axis.horizontal ? widget.thickness : null,
           height: widget.axis == Axis.vertical ? widget.thickness : null,
-          child: Center(
+          child: Align(
+            alignment: widget.slot == Slots.sidebar ? Alignment.centerRight : Alignment.centerLeft,
             child: Container(
               width: widget.axis == Axis.horizontal ? 1 : null,
               height: widget.axis == Axis.vertical ? 1 : null,