fix: Straighten bottom border of subnav

A fix for the issue described in this ticket - #22 (closed) - in which the border-radius applied to .Card elements was mistakenly cascading onto .Subnav, which should not have any border-radius. Explicitly defining a border-radius of 0 for .Subnav has fixed this.

Closes: #22 (closed)

static/sass/sections/_subnav.scss

@@ -5,6 +5,7 @@
   padding: 0;
   font-weight: 600;
   border-bottom: 3px solid palette.$borderNeutral;
+  border-radius: 0;
 
   ul {
     justify-content: center;