feat(chat): add "Mark as Unread" action and unread messages divider

- Add "Mark as Unread" option to message context menu in both
  contact and channel chats
- Show "New messages" divider line between read and unread messages
- Add setContactUnreadCount/setChannelUnreadCount methods to connector

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

lib/l10n/app_en.arb

@@ -824,6 +824,8 @@
       }
     }
   },
+  "chat_markAsUnread": "Mark as Unread",
+  "chat_newMessages": "New messages",
   "chat_openLink": "Open Link?",
   "chat_openLinkConfirmation": "Do you want to open this link in your browser?",
   "chat_open": "Open",

lib/l10n/app_localizations_en.dart

@@ -1527,6 +1527,12 @@ class AppLocalizationsEn extends AppLocalizations {
     return 'Unread: $count';
   }
 
+  @override
+  String get chat_markAsUnread => 'Mark as Unread';
+
+  @override
+  String get chat_newMessages => 'New messages';
+
   @override
   String get chat_openLink => 'Open Link?';