Function backend_close_window 

pub(super) fn backend_close_window(
    web_endpoint: &str,
    auth_key: &str,
    window_id: &str,
)

Synchronously tell the backend to close a window’s workspace/tabs/shells.

Uses a raw TCP connection so no async runtime or extra crate is needed. Called from a background thread in on_before_close so the CEF UI thread is not blocked. Fire-and-forget: we write the request and don’t read the response.