internal/ui: data race in tray icon handling

[DeedleFake]

https://github.com/DeedleFake/trayscale/blob/764e58c0d56fbb287a9b3835d44a7ade759c8bc8/internal/ui/app.go#L350-L352

https://github.com/DeedleFake/trayscale/blob/764e58c0d56fbb287a9b3835d44a7ade759c8bc8/internal/ui/app.go#L182

The above read and write of a.tray happen concurrently and are not synchronized. This is an extremely unlikely problem considering the way that initialization happens, but it's still technically a bug and should be fixed.