feat(subsonic): populate Folder field with user's accessible library IDs

Added functionality to populate the Folder field in GetUser and GetUsers API responses
with the library IDs that the user has access to. This allows Subsonic API clients
to understand which music folders (libraries) a user can access for proper
content filtering and UI presentation.

Signed-off-by: Deluan <deluan@navidrome.org>

server/subsonic/users.go

@@ -7,6 +7,7 @@ import (
 	"github.com/navidrome/navidrome/model"
 	"github.com/navidrome/navidrome/model/request"
 	"github.com/navidrome/navidrome/server/subsonic/responses"
+	"github.com/navidrome/navidrome/utils/slice"
 )
 
 // buildUserResponse creates a User response object from a User model
@@ -19,6 +20,7 @@ func buildUserResponse(user model.User) responses.User {
 		ScrobblingEnabled: true,
 		DownloadRole:      conf.Server.EnableDownloads,
 		ShareRole:         conf.Server.EnableSharing,
+		Folder:            slice.Map(user.Libraries, func(lib model.Library) int32 { return int32(lib.ID) }),
 	}
 
 	if conf.Server.Jukebox.Enabled {
@@ -28,7 +30,6 @@ func buildUserResponse(user model.User) responses.User {
 	return userResponse
 }
 
-// TODO This is a placeholder. The real one has to read this info from a config file or the database
 func (api *Router) GetUser(r *http.Request) (*responses.Subsonic, error) {
 	loggedUser, ok := request.UserFrom(r.Context())
 	if !ok {