fix(chat): open the conversation with the other participant

/chat/start/:id starts a chat between the caller and :id, so it takes the
counterpart — the web already passes the client when a professional opens it.
Mobile always sent the professional, so a professional opening the chat
started a conversation with themselves.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/screens/chat/chat_screen.dart

@@ -45,10 +45,18 @@ class _ChatScreenState extends State<ChatScreen> {
   }
 
   void _loadChat() {
+    // /chat/start/:id opens the conversation between the caller and :id, so it
+    // takes the *other* participant. Always sending the professional meant a
+    // professional opening the chat started a conversation with themselves.
+    final me = ApiUserRepository.currentUserId ?? '';
+    final counterpartId = me == widget.service.professionalId
+        ? widget.service.userId
+        : widget.service.professionalId;
+
     chatBloc.add(LoadChatEvent(
       serviceId: widget.service.id ?? '',
       userId: widget.service.userId,
-      professionalId: widget.service.professionalId,
+      professionalId: counterpartId,
     ));
   }