Add volume slider

[updownupdown]

I was looking over sheet music and wanted to use the BPM app (the song is tricky to keep track of, very fast arpeggios), but the sound is super loud even when I have Spotify on max, making is impossible to use. I'd add a slider at the bottom to control volume, defaults to 65% or something like that so it doesn't look like it's fully centered and a decorative element.

[greyscaled]

@updownupdown sounds good, can hopefully push something up this weekend