fix(chat): address mark-as-unread double-pop and missed map entry point

- Remove stray Navigator.pop(context) in _markAsUnread for both contact
  and channel chats so the action no longer exits the conversation
- Thread initialUnreadCount through map discovered-contact "Open Chat"
  button so the unread divider/jump still fires from that entry point

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/screens/channel_chat_screen.dart

@@ -400,7 +400,6 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
       if (found && !m.isOutgoing) count++;
     }
     connector.setChannelUnreadCount(widget.channel.index, count);
-    Navigator.pop(context);
   }
 
   Widget _buildMessageBubble(ChannelMessage message, double textScale) {

lib/screens/chat_screen.dart

@@ -543,7 +543,6 @@ class _ChatScreenState extends State<ChatScreen> {
       if (found && !m.isOutgoing && !m.isCli) count++;
     }
     connector.setContactUnreadCount(widget.contact.publicKeyHex, count);
-    Navigator.pop(context);
   }
 
   Widget _buildInputBar(MeshCoreConnector connector) {

lib/screens/map_screen.dart

@@ -1463,11 +1463,16 @@ class _MapScreenState extends State<MapScreen> {
                 if (!contact.isActive) {
                   connector.importDiscoveredContact(contact);
                 }
+                final unread =
+                    connector.getUnreadCountForContactKey(contact.publicKeyHex);
                 Navigator.pop(dialogContext);
                 Navigator.push(
                   context,
                   MaterialPageRoute(
-                    builder: (context) => ChatScreen(contact: contact),
+                    builder: (context) => ChatScreen(
+                      contact: contact,
+                      initialUnreadCount: unread,
+                    ),
                   ),
                 );
               },