🎨(frontend) replace logical OR with nullish coalescing operator

Replace `||` operators and conditional checks with `??` where appropriate
for safer null/undefined handling. Nullish coalescing only triggers for
null/undefined values, preventing unexpected behavior with falsy values
like 0 or empty strings.

src/frontend/src/components/Avatar.tsx

@@ -51,7 +51,7 @@ export const Avatar = ({
   style,
   ...props
 }: AvatarProps) => {
-  const initial = name?.trim()?.charAt(0) || ''
+  const initial = name?.trim()?.charAt(0) ?? ''
   return (
     <div
       style={{