feat: configurable menu bar position for mailbox view (#298)

Move the folder navigation drawer trigger to the bottom by default,
matching the issue request. Add a user preferences DB table (schema v34)
and a settings screen so users can switch back to the top hamburger menu.

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

lib/ui/screens/email_list_screen.dart

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