[globaldrawer]: Explicitly set height of header

In some cases (while I haven't figured out a minimal example, this can be reproduced in Discover), the header's height calculations can just completely nope out and go to 0, causing layout breakages. Since it's not in a layout, we're free to set the height to the sum of its content implicit heights.

Before:

After:

(Don't mind broken labels in after, unrelated Discover bug reproducible with before)