top-align sidebar and context panel content

Side panel content was vertically centered by Container's default
alignment. Added Alignment.topLeft so file tree, viewer, and other
panel content docks to the top.

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

app/lib/app.dart

@@ -299,6 +299,7 @@ class _SidebarSlot extends StatelessWidget {
     final tokens = ClideTheme.of(context).surface;
     return Container(
       color: tokens.sidebarBackground,
+      alignment: Alignment.topLeft,
       child: active.build(context),
     );
   }
@@ -414,6 +415,7 @@ class _ContextSlot extends StatelessWidget {
     final tokens = ClideTheme.of(context).surface;
     return Container(
       color: tokens.panelBackground,
+      alignment: Alignment.topLeft,
       child: active.build(context),
     );
   }