Migrate library endpoints to /me/library

[wardbox]

Context

Part of the Spotify Feb 2026 API changes (#70). The following endpoints are being consolidated:

• PUT/DELETE /me/tracks, /me/albums, /me/shows → PUT/DELETE /me/library (with Spotify URIs)
• GET /me/tracks/contains, /me/albums/contains, /me/shows/contains → GET /me/library/contains
• PUT/DELETE /me/following (artist/user) → PUT/DELETE /me/library
• GET /me/following/contains → GET /me/library/contains
• PUT/DELETE /playlists/{id}/followers → PUT/DELETE /me/library
• GET /playlists/{id}/followers/contains → GET /me/library/contains

Files affected (~19)

Library/

• Get-CurrentUserSavedTracks.ps1
• Add-CurrentUserSavedTrack.ps1
• Remove-CurrentUserSavedTrack.ps1
• Test-CurrentUserSavedTracks.ps1
• Get-CurrentUserSavedAlbums.ps1
• Add-CurrentUserSavedAlbum.ps1
• Remove-CurrentUserSavedAlbum.ps1
• Test-CurrentUserSavedAlbum.ps1
• Get-CurrentUserSavedShows.ps1
• Add-CurrentUserSavedShow.ps1
• Remove-CurrentUserSavedShow.ps1
• Test-CurrentUserSavedShow.ps1

Follow/

• Get-FollowedArtists.ps1
• Add-FollowedArtist.ps1, Remove-FollowedArtist.ps1, Test-FollowedArtist.ps1
• Add-FollowedUser.ps1, Remove-FollowedUser.ps1, Test-FollowedUser.ps1
• Add-FollowedPlaylist.ps1, Remove-FollowedPlaylist.ps1, Test-FollowedPlaylist.ps1

Tracks/

• Test-UserSavedTrack.ps1

Notes

• New /me/library endpoint uses Spotify URIs (e.g., spotify:track:xxx) instead of bare IDs
• Not urgent for existing apps (postponed by Spotify), but new apps are already affected