confirm the artist delete too

Only a single song deletes without asking; the artist's delete now opens the same
dialog as the album and playlist, naming the artist and the space it frees. The
artist's download action stays in its more-menu rather than getting a visible
control, deliberately, so bulk downloading a whole artist is a considered
action.

app/src/test/kotlin/com/subtracks/data/repo/DownloadRepositoryTest.kt

@@ -667,6 +667,7 @@ class DownloadRepositoryTest {
             writeFile(1, "s3", "123")
 
             assertEquals(5L, repository.downloadedBytes(1, DownloadList.Album, "al1"))
+            assertEquals(5L, repository.downloadedBytes(1, DownloadList.Artist, "ar1"))
         }
 
     private fun requestedSongIds(): List<String> = engine.requests.map { it.second.path.substringAfterLast('/') }