Handle missing Spotify auth gracefully instead of returning errors

Return nil/empty results when Spotify client is not authenticated,
preventing GraphQL errors from breaking the home page data query.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/graph/schema/schema.graphql

@@ -10,7 +10,7 @@ type Query {
   rowingSessions: [Rowing!]!
   messages: [Message!]!
   spotifyListening: SpotifyPlaying
-  spotifyRecent: [SpotifyRecentItem!]!
+  spotifyRecent: [SpotifyRecentItem!]
   giteaFeed: GiteaFeedItem
   me: User
 }