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.Infrastructure/Services/YouTubeMusicService.cs

@@ -162,7 +162,7 @@ namespace Musify.Infrastructure.Services
 
                 return new YouTubeAlbumDetail(
                     album,
-                    info.Description ?? string.Empty,
+                    StripWikipediaAttribution(info.Description),
                     (int)info.Duration.TotalSeconds,
                     tracks);
             }
@@ -173,6 +173,16 @@ namespace Musify.Infrastructure.Services
             }
         }
 
+        private static string StripWikipediaAttribution(string? description)
+        {
+            if (string.IsNullOrEmpty(description))
+                return string.Empty;
+
+            var markerIndex = description.IndexOf("From Wikipedia", StringComparison.OrdinalIgnoreCase);
+            var cleaned = markerIndex >= 0 ? description[..markerIndex] : description;
+            return cleaned.TrimEnd();
+        }
+
         private YouTubeAlbumResult ToAlbumResult(AlbumSearchResult album) =>
             new(
                 album.Id,