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/chat/types.ts

@@ -11,6 +11,7 @@ export interface Chat {
   description?: string | null;
   is_public?: boolean;
   is_saved?: boolean;
+  unread_count?: number;
   pinned_message_id?: number | null;
   created_at: string;
 }