Fix styling of "head first" dropdown in exported flamegraph

[Rperry2174]

It looks as though the styling for this specific dropdown comes from elsewhere in the app.

to validate all reuqired css is shipped to both webapp and flamegraph most likely add these imports https://github.com/pyroscope-io/pyroscope/blob/836fa97f126f8b7ebfb966bb52a97b5bdf179d83/webapp/sass/profile.scss#L9-L12 here https://github.com/pyroscope-io/pyroscope/blob/836fa97f126f8b7ebfb966bb52a97b5bdf179d83/webapp/sass/flamegraph.scss#L8

If it's easy enough we could probably go ahead and use the actual dropdown component here, but if thats out of scope we can do that separately