Platform glue: file dialogs (via rlobkit-dialogs) + autosave storage.
Pattern mirrors my repadio's player-platform: a thin crate gated on the
target (not a Cargo feature), with a non-blocking callback API that works
on every platform and a few blocking helpers reserved for desktop. Dialogs
go through rlobkit-dialogs so one crate serves every platform (native
backends on desktop, browser/Activity pickers on WASM/Android).