Use send_ctrlc to terminate child app process.

[azriel91]

• Closes dx serve does not gracefully shutdown server on windows #5157.

• Also suppresses this warning when stopping the dx cli:

  WARN Failed to send devserver message to client (build_id: Some(BuildId(1)), pid: Some(24552)): IO error: An existing connection was forcibly closed by the remote host. (os error 10054)
  

[nicoburns]

This has issues for Dioxus Desktop on Windows:

1. Windows desktop regression risk (main concern). Old path: taskkill /PID (no /F) sends WM_CLOSE to the app window → tao close → persist_window_state(). New path: CTRL_BREAK_EVENT → Rust has no default handler → ExitProcess, so no WM_CLOSE, no state save. connect_preserve_window_state_handler in app.rs:655-680 is #[cfg(unix)] only ("TODO: make this work on windows"), so nothing catches the console event. Worse, for apps built with windows_subsystem = "windows" (no console) GenerateConsoleCtrlEvent just fails; the error is ignored (_ = process.terminate()), then after the 1s wait kill_on_drop does a hard TerminateProcess. Suggested fixes: (a) on Windows fall back to taskkill when terminate() errs, or send both; and/or (b) add a SetConsoleCtrlHandler (the ctrlc crate handles CTRL_BREAK) in desktop's handler that fires UserWindowEvent::Shutdown — that would actually resolve the TODO and make the fix complete for desktop, not just fullstack servers/web (which is what dx serve does not gracefully shutdown server on windows #5157 is about).

2. Nit: the doc comment in soft_kill still says "join the wait with a 100ms timeout" while the sleep is 1000ms (pre-existing); the new kill_on_drop comment is accurate since process is dropped at the end of soft_kill. The server.rs change (warn→debug for Shutdown) is fine; slightly cleaner would be to not log at all for Shutdown, but no objection.

[azriel91]

cool; I think those are fixed -- I tested using dx serve --hot-patch false --example window_zoom --desktop --features=desktop and ctrl-c'ed in the dx terminal.

I suppressed some taskkill output, though it only suppresses English (not sure if taskkill outputs other languages in different language Windows).