feat(artist): sort for the in-library albums and a split play button on every album section

The "In library" Albums section gains the release buckets' sort select
(same five keys, persisted by artist_prefs under the pseudo release type
"library"; rows sorted in QML off the document's seed). Every album
section — each catalog bucket and the library grid — gains a split
button right of its sort (controls/QbzSplitButton.qml, sized like the
sm select): the body plays the whole section in its sort order as one
queue replacing the current one (`QbzPlayer.playAlbums` ->
`playback_qt::play_albums`, each album resolved through the single-album
path, failures skipped), the chevron offers Play random (one album of
the section) and Play selected, which arms a per-section select mode:
the cards show their check bottom-right inside the art
(`AlbumCard.selectMarkBottom`), the button reads "Play selected (n)" and
plays the checked albums in sort order, and the menu swaps that row for
Cancel selection. One section is armed at a time; changing artist or
tab disarms. No queue options on purpose.

cc

crates/qbz-qt/build.rs

@@ -578,6 +578,7 @@ fn main() {
                 "qml/controls/ScopePanel.qml",
                 "qml/controls/QbzSegToggle.qml",
                 "qml/controls/QbzSelect.qml",
+                "qml/controls/QbzSplitButton.qml",
                 "qml/controls/QbzSlider.qml",
                 "qml/controls/QbzTabBar.qml",
                 "qml/controls/QbzTextArea.qml",