use serverTabPreferDiskAmount

[dsvf]

Hi! For my storage servers, I would prefer the server overview to show the Disk Amount instead of Disk IO. I have found the setting "serverTabPreferDiskAmount" as a fallback in tab.dart but there is no way to control it. Could you add a slider to the Settings screen for this?

Alternatively, if every server tile could remember whether it showed Disk Amount or Disk IO and apply this setting on app restart, that would also work. I'm not sure how or where you would store/persist this information (Spi.custom?) so I won't attempt a PR, but let me know if I can help you with this in any way.