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/widgets/unread_divider.dart

@@ -0,0 +1,32 @@
+import 'package:flutter/material.dart';
+
+import '../l10n/l10n.dart';
+
+class UnreadDivider extends StatelessWidget {
+  const UnreadDivider({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final color = Theme.of(context).colorScheme.primary;
+    return Padding(
+      padding: const EdgeInsets.symmetric(vertical: 8),
+      child: Row(
+        children: [
+          Expanded(child: Divider(color: color)),
+          Padding(
+            padding: const EdgeInsets.symmetric(horizontal: 8),
+            child: Text(
+              context.l10n.chat_newMessages,
+              style: TextStyle(
+                color: color,
+                fontSize: 12,
+                fontWeight: FontWeight.w500,
+              ),
+            ),
+          ),
+          Expanded(child: Divider(color: color)),
+        ],
+      ),
+    );
+  }
+}