refactor: backend-only transpose, remove client-side transpose logic

crates/presentation/src/routes/songs.rs

@@ -155,6 +155,13 @@ pub async fn get_song(
         song
     };
 
+    let song = match params.transpose {
+        Some(semitones) if semitones != 0 => {
+            ChordTransposer.transpose_song(&song, semitones)
+        }
+        _ => song,
+    };
+
     Ok(Json(song))
 }