💄(frontend) render Avatar initials in uppercase

Uppercase the initials rendered in the Avatar so their vertical
centering stays consistent.

With lowercase letters, the initials were slightly shifted toward
the bottom of the Avatar, which broke the alignment.

src/frontend/src/components/Avatar.tsx

@@ -56,7 +56,7 @@ export const Avatar = React.memo(
             fontWeight="500"
             fill="currentColor"
           >
-            {initial}
+            {initial.toUpperCase()}
           </text>
         </svg>
       </div>