improved library scan speeds, added library manager component

-libraries tab now opens a library manager
-choose library directory, manually soft scan and rescan
-batched db queues
-mp3 header-based duration extraction

backend/database/sql/queries/cover_art.sql

@@ -26,3 +26,6 @@ WHERE id = ?;
 -- name: DeleteCoverArt :exec
 DELETE FROM cover_art 
 WHERE id = ?;
+
+-- name: DeleteAllCoverArt :exec
+DELETE FROM cover_art;