[VSA] implement shadow loader and address timing issues with data load

Problem

In a walk through video @ebrinkman enchanted an error message that quickly appeared and disappeared saying something about calculating medians.

Proposal

• Implement a shadow loader while we are waiting for data to load.
• Also look for a bug related to callbacks because there appears to be a weird timing issue in addition to the lack of a good ux for alerting the user that data is loading.