fix side panel padding: no pad on divider edge

Sidebar pads left/top only (not right — that's the divider side).
Context panel pads right/top only (not left — divider side). The
divider handle's transparent pixels now show the center column's
background correctly.

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

lib/app.dart

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