android: the downloads say so in the shade

A Save still running with the app in the background now posts a progress
notice — how many files, the one it is on, a bar, Stop all and Open — and
takes it down when the queue empties. It sits on its own low-importance
channel so it can be silenced without silencing the player's card, and
rides its own dataSync foreground service rather than the media one: a
mediaPlayback service stops itself when the bar empties, a download is not
playback, and nothing else was keeping the process alive for a Save with
nothing playing — which is how the first real Save died, four tracks in.
Stop all goes through the same cancel(id) a row's tap takes, and Transfers
grew the matching button beside Clear finished. The walk pushes two rows
straight into the transfer store and reads the real notification back out
of `dumpsys notification`, without fetching anything.

src-tauri/src/lib.rs

@@ -85,6 +85,10 @@ pub fn run() {
             app.handle().plugin(tauri_plugin_storage::init())?;
             // The share sheet on the phone: a link sent from another app lands in the import.
             app.handle().plugin(tauri_plugin_share::init())?;
+            // The downloads in the phone's shade: a progress notice on its own
+            // channel and its own dataSync service, so a Save survives the app
+            // going to the background with nothing playing.
+            app.handle().plugin(tauri_plugin_transfer_notice::init())?;
             #[cfg(desktop)]
             app.handle().plugin(tauri_plugin_updater::Builder::new().build())?;
             // Must run before anything touches the app-data dir.