fix: wrap bottom bar menu button in Builder to get Scaffold context

Scaffold.of() requires a descendant context of the Scaffold widget.
Using the State's build context (which is the Scaffold's parent) caused
an assertion failure when tapping the 'Open folders' button.

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

lib/ui/screens/email_list_screen.dart

@@ -313,10 +313,12 @@ class _EmailListScreenState extends ConsumerState<EmailListScreen> {
     return BottomAppBar(
       child: Row(
         children: [
-          IconButton(
-            icon: const Icon(Icons.menu),
-            tooltip: 'Open folders',
-            onPressed: () => Scaffold.of(context).openDrawer(),
+          Builder(
+            builder: (context) => IconButton(
+              icon: const Icon(Icons.menu),
+              tooltip: 'Open folders',
+              onPressed: () => Scaffold.of(context).openDrawer(),
+            ),
           ),
         ],
       ),