feat: bring over the albums, explore and player work from the worktree

Moves the remaining uncommitted work: the home gets quick mixes and a
"Tus álbumes" shelf, saving a YouTube album now goes through
addYoutubeAlbumToPlaylistAction instead of the SaveYouTubeAlbum command,
and UserAlbum drops its unused ExternalId.

The mixes section keeps its current shape (no refresh action, no badge,
hidden when empty) rather than the older version the worktree carried.

backend/Musify.Api/Endpoints/AlbumEndpoints.cs

@@ -50,15 +50,6 @@ public static class AlbumEndpoints
             .Produces(StatusCodes.Status401Unauthorized)
             .Produces(StatusCodes.Status404NotFound);
 
-        group.MapPost("/youtube/{albumId}", SaveYouTubeAlbum)
-            .WithName("SaveYouTubeAlbum")
-            .WithSummary("Save A YouTube Music Album To Your Library With All Of Its Tracks.")
-            .RequireAuthorization()
-            .Produces<AlbumApplicationResponse>(StatusCodes.Status201Created)
-            .Produces(StatusCodes.Status401Unauthorized)
-            .Produces(StatusCodes.Status404NotFound)
-            .Produces(StatusCodes.Status409Conflict);
-
         group.MapPut("/{albumId}", UpdateAlbum)
             .WithName("UpdateAlbum")
             .WithSummary("Update An Album.")
@@ -152,19 +143,6 @@ public static class AlbumEndpoints
         return result.ToCreatedResult(album => $"/albums/{album.Id}");
     }
 
-    private static async Task<IResult> SaveYouTubeAlbum(
-        IMediator mediator,
-        CurrentUser currentUser,
-        string albumId,
-        CancellationToken cancellationToken)
-    {
-        var result = await mediator.Send(
-            new SaveYouTubeAlbumCommand(currentUser.RequiredId, albumId),
-            cancellationToken);
-
-        return result.ToCreatedResult(album => $"/albums/{album.Id}");
-    }
-
     private static async Task<IResult> UpdateAlbum(
         IMediator mediator,
         CurrentUser currentUser,