download a whole album, playlist or artist at once

Third block of #65: bulk downloads and the list-level actions.

- The hero's download button on albums and playlists is real now. It starts the
  songs the list is missing, cancels what is in flight, or deletes what is
  already downloaded, chosen from the list's own status. The artist has no hero
  button, so its bulk actions live in the context menu, which now carries the
  same status.
- No new storage: a bulk job is the download rows for the list, and the status is
  counted from song_downloads joined to the list's songs, so cancel and delete
  stay well defined without anything to keep in sync.
- downloadAll fetches artwork for the whole list under one shared budget, once
  per unique cover, and skips songs that are already downloaded so re-tapping
  Download resumes rather than restarting.

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

@@ -5,6 +5,8 @@ import androidx.lifecycle.viewModelScope
 import com.subtracks.data.model.Album
 import com.subtracks.data.model.CoverArtRef
 import com.subtracks.data.model.Disc
+import com.subtracks.data.model.DownloadList
+import com.subtracks.data.model.ListDownloadStatus
 import com.subtracks.data.model.Song
 import com.subtracks.data.model.SongDownload
 import com.subtracks.data.repo.DownloadRepository
@@ -13,10 +15,12 @@ import com.subtracks.data.repo.SourceRepository
 import com.subtracks.playback.PlaybackController
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.filterNotNull
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.flow.flatMapLatest
+import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.launch
 
 @OptIn(ExperimentalCoroutinesApi::class)
@@ -37,6 +41,26 @@ class AlbumDetailViewModel(
 
     val downloads: StateFlow<Map<String, SongDownload>> = downloadRepository.states()
 
+    val downloadStatus: StateFlow<ListDownloadStatus> =
+        sourceId
+            .flatMapLatest { downloadRepository.status(it, DownloadList.Album, albumId) }
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), ListDownloadStatus())
+
+    fun downloadAll() {
+        viewModelScope.launch { downloadRepository.downloadAll(sourceId.first(), DownloadList.Album, albumId) }
+    }
+
+    fun cancelDownloads() {
+        viewModelScope.launch { downloadRepository.cancelAll(sourceId.first(), DownloadList.Album, albumId) }
+    }
+
+    fun deleteDownloads() {
+        viewModelScope.launch {
+            downloadRepository.deleteAll(sourceId.first(), DownloadList.Album, albumId)
+            playbackController.refreshMediaItems()
+        }
+    }
+
     fun coverArt(
         coverArt: String?,
         thumbnail: Boolean,