FIX: Ensure floating quote button is distanced from iOS selection (PR #15588)

Tapping within ~50px of the selection end handle on iOS doesn’t trigger a click event. This commit ensures that our quote buttons are always at least 50px away from the end handle. It will try 4 positions in order of preference:

  1. The original position
  2. 50px to the left of the handle
  3. 50px to the right of the handle
  4. 50px below the handle, centered on the handle