Fix AvatarHeadDecapComponent reactor cleanup

[speigg]

Essentially, this is an anti-pattern:

useEffect(() => {
  if (condition) {
    // do something
  } else {
    // cleanup
  }
})

Because the cleanup code does not necessarily run when the reactor is stopped. The correct pattern is:

useEffect(() => {
  if (condition) {
    // do something
    return () => { 
      // cleanup 
    }
  }
})

Also updates hasComponent to be based on Component.existenceMap as source of truth

References

closes #insert number here

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