fix: use dialog context in delete confirmation to prevent shell nav pop

Navigator.pop(outerContext) inside a showDialog builder resolves to the
ShellRoute's nested navigator instead of the root navigator where the
dialog lives, popping the conversations route and showing a black screen.

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

lib/screens/chat/conversations_tab_screen.dart

@@ -99,15 +99,15 @@ class ConversationsTabScreen extends ConsumerWidget {
       BuildContext context, WidgetRef ref, int id, String title) async {
     final ok = await showDialog<bool>(
       context: context,
-      builder: (_) => AlertDialog(
+      builder: (dialogContext) => AlertDialog(
         title: const Text('Delete conversation?'),
         content: Text('"$title" will be permanently deleted.'),
         actions: [
           TextButton(
-              onPressed: () => Navigator.pop(context, false),
+              onPressed: () => Navigator.pop(dialogContext, false),
               child: const Text('Cancel')),
           FilledButton(
-              onPressed: () => Navigator.pop(context, true),
+              onPressed: () => Navigator.pop(dialogContext, true),
               child: const Text('Delete')),
         ],
       ),