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

app/chats/schemas.py

@@ -16,6 +16,7 @@ class ChatRead(BaseModel):
     description: str | None = None
     is_public: bool = False
     is_saved: bool = False
+    unread_count: int = 0
     pinned_message_id: int | None = None
     created_at: datetime