bound the downloads handed to the platform, and confirm bulk deletes

Two things a 3k-song playlist turned up, plus the hero's downloaded state.

- The platform runs about eight downloads at a time, but a bulk action handed it
  every song at once. Three thousand pending requests meant a notification flood,
  and a poll tick that rewrote every row's bytes/total, so the whole list churned
  ten times a second. Now a bulk action only records the songs and hands over at
  most IN_FLIGHT_LIMIT, oldest first, promoting the next as each finishes, so our
  row state mirrors what is actually running and a cancel only has to reach what
  is in flight.
- Cancelling was therefore unusable at that size. It now touches at most the
  in-flight requests instead of thousands, and the rows wait behind it.
- A queued row with no engine id is waiting its turn, not lost, so reconcile no
  longer re-enqueues the whole list on every pass; a row whose engine entry
  vanished goes back to waiting and the next promote picks it up. A song that
  cannot be handed over fails instead of being retried every tick.
- Requests follow the order the screen shows: artist songs resolve through the
  same albums ordering the artist page uses, so they were the one list that did
  not.
- The hero's fully-downloaded state shows the downloaded glyph rather than a bin,
  and tapping it asks before deleting, naming the album or playlist and the space
  it frees.

app/src/main/kotlin/com/subtracks/ui/library/AlbumDetailViewModel.kt

@@ -47,6 +47,8 @@ class AlbumDetailViewModel(
             .flatMapLatest { downloadRepository.status(it, DownloadList.Album, albumId) }
             .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), ListDownloadStatus())
 
+    suspend fun downloadedBytes(): Long = downloadRepository.downloadedBytes(sourceId.first(), DownloadList.Album, albumId)
+
     fun onDownloadAction(action: BulkDownloadAction) {
         viewModelScope.launch {
             downloadRepository.applyAction(sourceId.first(), DownloadList.Album, albumId, action)