refactor: use tailwind classes for App-header

[yosevu]

This PR adds Tailwind CSS and refactors the App-header styles to use Tailwind utility classes.

.App-header {
  background-color: #282c34;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  font-size: calc(10px + 2vmin);
  color: white;
}

<div className="App">
  <header className="bg-gray-800 min-h-screen flex flex-col items-center justify-center text-white text-lg">
    <Logo className="App-logo" id="App-logo" title="React logo" />
    <p>Hello, World!</p>
    <p>I'm a Chrome Extension Popup!</p>
  </header>
</div>

[nkilm]

@yosevu Hey, it would be helpful if you add extend the tailwindcss to content-script also. Thank you!