Support selecting Navidrome/OpenSubsonic music folders

[pimenov]

Feature request

Please add first-class support for Subsonic/OpenSubsonic music folders (Navidrome libraries), with a user-selectable library scope.

Real-world use case

My Navidrome server intentionally exposes three separate libraries through getMusicFolders:

• a large archive of rare DJ sets;
• a normal listener music library;
• my own releases and DJ mixes.

Cassette currently presents all three as one combined collection. That is useful as an All Libraries view, but on both macOS and iOS I also need to browse and search within one selected library. Other Subsonic clients can expose this server-side separation to the user.

Expected UX

• Fetch accessible folders with getMusicFolders.
• Provide an All Libraries default plus selection of one or multiple folders.
• Persist the selection per configured server.
• Apply the selected scope consistently to Home, Artists, Albums, Songs, Search, Recently Added, random/shuffle and discovery views where the Subsonic endpoint supports musicFolderId.
• Keep server-global items such as playlists working normally when they cannot be scoped reliably.
• Clearly show the active folder scope in the sidebar/header, especially on macOS.

Keeping All Libraries as the default would preserve today's behavior and compatibility with servers that expose only one folder.

Current behavior / implementation observation

On the current main revision I could not find a getMusicFolders call or a musicFolderId parameter in Cassette. LibraryService calls endpoints such as getArtists, getAlbumList2, search3, and getRandomSongs without a folder scope, so a Navidrome multi-library server is necessarily aggregated.

Environment

• Cassette for macOS
• Navidrome with multiple libraries exposed through the Subsonic/OpenSubsonic API
• The same account can access all three libraries

This would make Cassette work especially well for curated self-hosted collections where archives, everyday music, and owner-created material must stay logically separate while remaining on one Navidrome server.