fix: use Builder to get descendant context for Scaffold.of() in bottom nav

Calling Scaffold.of(context) with the widget's own build context crashes
when no ancestor Scaffold exists in that context. Wrapping the IconButton
in a Builder provides a child context that is a proper descendant of the
Scaffold, which is what Scaffold.of() requires.

Closes #397

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/screens/mailbox_list_screen.dart

@@ -51,10 +51,12 @@ class MailboxListScreen extends ConsumerWidget {
           ? BottomAppBar(
               child: Row(
                 children: [
-                  IconButton(
-                    icon: const Icon(Icons.menu),
-                    tooltip: 'Open folders',
-                    onPressed: () => Scaffold.of(context).openDrawer(),
+                  Builder(
+                    builder: (ctx) => IconButton(
+                      icon: const Icon(Icons.menu),
+                      tooltip: 'Open folders',
+                      onPressed: () => Scaffold.of(ctx).openDrawer(),
+                    ),
                   ),
                 ],
               ),