message: fix(core): fix sheet in no browser
diff: import { isBrowser } from '@fower/utils'
  class StyleSheet {
  - private $style: HTMLStyleElement | null = globalThis.document?.querySelector?.('[data-fower]')
  + constructor() {
  + if (isBrowser) {
  + this.$style = globalThis.document?.querySelector?.('[data-fower]')
  + }
  + }
  + private $style: HTMLStyleElement | null = null
  private cssString = ''
  