Finding 1: the Rust side calls on_netplay_resume_session,
on_netplay_save_session and set_netplay_is_host on AppWindow, but the
callbacks/property were only declared on the NetplayCreate/NetplayWait
sub-components. appwindow.slint now declares callback
netplay_resume_session(string), callback netplay_save_session and the
in-out property netplay_is_host <=> NetplayWaitData.is_host, and
forwards them at the NetplayCreate/NetplayWait instantiation sites,
matching the existing netplay callback pattern.
Finding 2: rfd::AsyncFilePicker does not exist in rfd 0.17; the crate
exports AsyncFileDialog (already used by this repo in src/ui/gui.rs).
Switch the resume file picker to rfd::AsyncFileDialog and use
FileHandle::path() for the display path.
Finding 3: set is_host from the ResponseSession lobby update instead of
at wait-window open, comparing the local player name against players[0]
(the session creator, always registered first). Re-evaluates on every
lobby update.
