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/build.rs

@@ -1,3 +1,8 @@
+// tauri-build only rewrites gen/android/tauri.settings.gradle and
+// app/tauri.build.gradle.kts — the lines that include every plugin's own
+// Android module — when this build script re-runs, so touch this file
+// whenever a plugin is added (ANDROID.md). The downloads notice was the
+// last one.
 fn main() {
     tauri_build::build()
 }