Set map element height more correctly

[mileswwatkins]

Right now, it's a hack, and not even one that's accurate for most browsers. Eg, on Brave on Windows the page is larger than 100vh vertically and so the entire page scrolls.

        style={{
          width: "100%",
          // This is still a hack, but represents the full page height minus
          // the header text and padding and the hairline border between panes
          height: "calc(100vh - 38px - 2.5rem - 1px)",
        }}