feat(web-core): implement unread counters and new-messages divider

backend:

- add unread_count to ChatRead serialization

- compute unread_count per chat using message_receipts and hidden messages

web:

- add unread badges in chat list

- track unread boundary per chat in store

- show 'New messages' divider in message list

- update realtime flow to increment/clear unread on incoming events

web/src/hooks/useRealtime.ts

@@ -69,6 +69,9 @@ export function useRealtime() {
             ws.send(JSON.stringify({ event: "message_delivered", payload: { chat_id: chatId, message_id: message.id } }));
             if (chatId === chatStore.activeChatId) {
               ws.send(JSON.stringify({ event: "message_read", payload: { chat_id: chatId, message_id: message.id } }));
+              chatStore.clearUnread(chatId);
+            } else {
+              chatStore.incrementUnread(chatId);
             }
           }
           if (!chatStore.chats.some((chat) => chat.id === chatId)) {