feat(explore): give the album page a primary action that tells the truth

H-13: no Play, no Shuffle, no Add to queue on the album header. The
reason it is not just three buttons is that explore-album-details is a
catalog page — there is no library-side album detail page at all — so
the album shown may be wholly the user's, partly theirs, or not theirs.
A Play button that plays 7 of a 40-track release under a label saying
'Play' is the page lying about what is owned, so the button says which:
'Play' when all of it is owned, 'Play 7 of 12' when some is, and no
play button at all when none is.

albumLibraryStatus() stays as it was — four claims of decreasing
confidence OR'd into one tick, the weakest firing when a single
recording matches. That is a fine answer to 'is any of this mine' and a
useless basis for a button, so ownership() counts the displayed
tracklist instead.

GetFilePathsByRecordingMBIDs is the catalog-side sibling of
GetFilePathsByAlbums: one query, paths only, grouped so the caller
keeps the tracklist's order. It is keyed on recording MBID because that
is how the backend decides a track is inLibrary, and because
MBTrack.LocalID is declared and never written by anything. The local
album id is preferred where there is one — a library-only album has no
MBIDs at all, and keying on them alone queued nothing.

The ticks also get the legend H-13 asks for. They were never unlabelled
— the indicator has carried a title and aria-label all along — but a
sighted user got a column of green circles and no key.

backend/database/sql/queries/audio_files.sql

@@ -318,3 +318,28 @@ JOIN audio_files af ON af.recording_id = r.id
 WHERE rgr.release_group_id IN (sqlc.slice('release_group_ids'))
   AND af.library_id = ?
 ORDER BY rgr.disc_number, rgr.track_number;
+
+-- Same shape again, keyed on recording MBID, for the catalog side.
+-- An Explore album page knows which of its tracks the user owns only
+-- as a set of recording MBIDs -- that is exactly how the backend
+-- decides `inLibrary` (markReleasesInLibrary -> CheckMBIDs) -- and
+-- MBTrack.LocalID is declared but never written by anything, so there
+-- is no id to ask by. Grouped by MBID because a recording can have
+-- more than one file (the duplicate fixtures are precisely that) and
+-- because the caller owns the order: the tracklist's, not the
+-- database's.
+
+-- name: GetFilePathsByRecordingMBIDs :many
+SELECT r.mbid AS recording_mbid, af.file_path
+FROM recordings r
+JOIN audio_files af ON af.recording_id = r.id
+WHERE r.mbid IN (sqlc.slice('mbids'))
+ORDER BY af.file_path;
+
+-- name: GetFilePathsByRecordingMBIDsByLibrary :many
+SELECT r.mbid AS recording_mbid, af.file_path
+FROM recordings r
+JOIN audio_files af ON af.recording_id = r.id
+WHERE r.mbid IN (sqlc.slice('mbids'))
+  AND af.library_id = ?
+ORDER BY af.file_path;