style: the play button is green

It was the same red as the recording indicator beside it, so at that size the
two read as one control. Green is also what the theme already uses for a
go-ahead action (the amplifier ON buttons), and it is a semantic token, so it
follows all four themes rather than pinning a colour.

frontend/src/App.tsx

@@ -3741,7 +3741,7 @@ export default function App() {
             onMouseDown={(e) => e.preventDefault()}
             onClick={playRecordingOnAir}
             title={t('rec.playOnAir')}
-            className="inline-flex items-center justify-center size-4 rounded text-destructive hover:bg-destructive-muted"
+            className="inline-flex items-center justify-center size-4 rounded text-success hover:bg-success/15"
           >
             ▶
           </button>