fix: create conversations with empty title so server auto-names them

The app was creating conversations with title 'New conversation'.
The server only generates a title when conv_title is falsy (empty).
With a non-empty title, should_gen_title is False for the first
message (msg_count % 10 != 0), so auto-naming never fired.

Now creates with empty title (matching web app behaviour). The list
still displays 'New conversation' as a UI placeholder until the
server-generated title arrives.

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

lib/screens/chat/conversations_tab_screen.dart

@@ -23,7 +23,7 @@ class ConversationsTabScreen extends ConsumerWidget {
             onPressed: () async {
               final conv = await ref
                   .read(conversationsProvider.notifier)
-                  .create('New conversation');
+                  .create('');
               if (context.mounted) {
                 context.push(Routes.chat.replaceFirst(':id', '${conv.id}'));
               }
@@ -52,7 +52,7 @@ class ConversationsTabScreen extends ConsumerWidget {
                     onPressed: () async {
                       final conv = await ref
                           .read(conversationsProvider.notifier)
-                          .create('New conversation');
+                          .create('');
                       if (context.mounted) {
                         context.push(
                             Routes.chat.replaceFirst(':id', '${conv.id}'));
@@ -71,8 +71,10 @@ class ConversationsTabScreen extends ConsumerWidget {
                 final c = convs[i];
                 return ListTile(
                   leading: const Icon(Icons.chat_bubble_outline),
-                  title: Text(c.title,
-                      maxLines: 1, overflow: TextOverflow.ellipsis),
+                  title: Text(
+                      c.title.isEmpty ? 'New conversation' : c.title,
+                      maxLines: 1,
+                      overflow: TextOverflow.ellipsis),
                   subtitle: Text(
                     _relativeTime(c.updatedAt),
                     style: theme.textTheme.labelSmall,