revert(a11y): drop bold weight on active sidebar nav link

The bold weight looked visually heavy. Leaving the color-only
indicator for now; a better non-color cue will land later.

Author: kolaente

frontend/src/styles/theme/navigation.scss

@@ -107,7 +107,6 @@
 
         &.router-link-exact-active {
           color: var(--primary);
-          font-weight: 700;
         }
 
         .icon {