Signal transfer cancel synchronously before any runtime await, drive
API calls via Handle::block_on so concurrent cancel cannot deadlock a
receive, and rewrite the gated output-sink cancel test to avoid the
slow multi-megabyte write loop that starved CI.
Attribute provider transfer progress to each remote endpoint so the
receivers panel and catalog can show per-peer send bars for multi-file
collections while a share is active.
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.
