Add a signal to keep direct-message chat name and icon in sync when a user updates their username or avatar, and register it in AppConfig.ready. Also initialize a DM chat's name/icon on creation from the other member's username/avatar if those fields are missing, saving only the changed fields.
Expanded chat backend with message reply, edit, delete, and reaction support in consumers and models. Updated routing to use chat_id. Added chat-related view stubs. On the frontend, introduced ChatLayout and ChatPage scaffolding, and routed protected routes through ChatLayout.
