[Feature Request] Custom sort order for playlists, returned by getPlaylists in that order

Is your feature request related to a problem?

When using third-party Subsonic clients (mobile apps, alternative web clients) to browse playlists, the playlists are always returned in the server's default order (by name / updated time). There is no way to define a personal order — for example, pinning frequently-used playlists to the top — and have that order preserved across all clients.

Describe the solution you'd like

Allow users to define a custom sort order for their playlists in the Navidrome admin/web UI (e.g. drag-and-drop reordering). The admin UI order is the source of truth, and getPlaylists simply returns the playlists in that order — no new API sorting parameters or custom-sort support on the API side is needed.

• Per-user custom ordering of the playlist list, configured in the admin UI
• getPlaylists returns the playlists in the user-defined order (no API change required, just return the sorted list)
• The web UI and third-party clients share the same ordering

Describe alternatives you've considered

• Sorting by name / last-played / created on the client — each client ends up with its own order, and a manual order set in the web UI is lost.
• Adding custom-sort support directly to the Subsonic API (sort params etc.) — overkill; the admin UI order is enough if the API just returns it in that order.

Additional context

This would make playlist ordering consistent between the built-in web UI and third-party Subsonic clients, without requiring any API-level sorting features.