Make the transfer loop feel live: parse real core progress payloads, show
progress on send/receive surfaces, allow cancel/stop, retry failed receives,
and fix delivery-phase events so receiver completion updates the UI.
Default Android receive destination is now the shared system Downloads
collection (like desktop), using MediaStore on API 29+ instead of
app-private storage. Legacy app-private defaults are promoted back to
public Downloads so existing installs pick up the fix.
Android emulated storage rejects hard-link commit even when canWrite()
reports success. Fall back to exclusive rename for publish, probe real
writes during folder validation, and label the default app downloads dir
clearly so it is not confused with shared system Downloads.
