fix(library): open track details by path, not from the whole library

Track details from the queue, a playlist or a smart playlist looked
the track up in libraryStore's whole-library array, and returned
silently when it had not loaded, so the menu item did nothing. That
dependency is also why every track was fetched eagerly at startup
(20.5 MB at 26k tracks).

GetTracksByPaths answers for the paths in hand, and trackCache holds
the answers: bounded, coalesced into one call per task, and forgetting
what the retag, removal and scan events name. Every opener, and
track-details' own refresh after a save, go through it. A source sweep
pins the whole-library array to the store and the Tracks view.

Closes #279

backend/database/sql/queries/audio_files.sql

@@ -117,6 +117,9 @@ WHERE library_id = COALESCE(NULLIF(CAST(sqlc.arg(library_id) AS INTEGER), 0), li
 -- name: GetTrackByPath :one
 SELECT * FROM track_metadata WHERE file_path = ? LIMIT 1;
 
+-- name: GetTracksByPaths :many
+SELECT * FROM track_metadata WHERE file_path IN (sqlc.slice('paths'));
+
 -- name: GetTracksByAlbum :many
 SELECT * FROM track_metadata
 WHERE album_id = sqlc.arg(album_id)