Fix spoiler toggle button being able to submit compose form (#36736)

app/javascript/mastodon/components/status_banner.tsx

@@ -49,6 +49,7 @@ export const StatusBanner: React.FC<{
 
       <button
         ref={buttonRef}
+        type='button'
         className='link-button'
         onClick={onClick}
         aria-describedby={descriptionId}