add the Downloaded filter to the library tabs

One toggle per tab, persisted with the rest of the list query, narrowing to
entities that have at least one completed download. It rides the existing filter
SQL: every album, artist and playlist paging query takes a flag and the shared
filter constant adds an EXISTS over song_downloads, so it composes with the
sort, search and starred filter rather than being a separate mode.

The filters sheet now renders on every tab, since playlists support Downloaded
but not Starred.

app/src/main/kotlin/com/subtracks/data/prefs/UserPreferences.kt

@@ -35,6 +35,7 @@ data class ListQuery(
     val sort: String,
     val descending: Boolean = false,
     val starred: StarredFilter = StarredFilter.Any,
+    val downloaded: Boolean = false,
 )
 
 data class StreamQuality(
@@ -107,7 +108,8 @@ class UserPreferences(
 
     private fun listQueryKey(tab: LibraryListTab) = stringPreferencesKey("list_query_${tab.key}")
 
-    private fun encode(query: ListQuery) = "${query.sort}|${if (query.descending) 1 else 0}|${query.starred.ordinal}"
+    private fun encode(query: ListQuery) =
+        "${query.sort}|${if (query.descending) 1 else 0}|${query.starred.ordinal}|${if (query.downloaded) 1 else 0}"
 
     private fun encode(quality: StreamQuality) = "${quality.maxBitrate}|${quality.format.orEmpty()}"
 
@@ -130,11 +132,12 @@ class UserPreferences(
         defaultSort: String,
     ): ListQuery {
         val parts = stored?.split("|")
-        if (parts == null || parts.size != 3) return ListQuery(defaultSort)
+        if (parts == null || parts.size < 3) return ListQuery(defaultSort)
         return ListQuery(
             sort = parts[0].ifEmpty { defaultSort },
             descending = parts[1] == "1",
             starred = StarredFilter.entries.getOrElse(parts[2].toIntOrNull() ?: 0) { StarredFilter.Any },
+            downloaded = parts.getOrNull(3) == "1",
         )
     }
 }