fix(frontend): resolve button hydration mismatch with undefined variant/size (#1506)

Server-rendered data-variant={undefined} didn't match client hydration.
Now only render data-variant and data-size when explicitly set.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: JeffJiang <for-eleven@hotmail.com>

frontend/src/components/ui/button.tsx

@@ -52,8 +52,8 @@ function Button({
   return (
     <Comp
       data-slot="button"
-      data-variant={variant}
-      data-size={size}
+      {...(variant !== undefined && { "data-variant": variant })}
+      {...(size !== undefined && { "data-size": size })}
       className={cn(buttonVariants({ variant, size, className }))}
       {...props}
     />