Basic accessibility like keyboard navigation (on the public facing slides)

[BogdanCerovac]

Describe the bug
Currently only mouse reveals buttons in the (bottom) navigation, if user wants to use keyboard buttons remain hidden.

To Reproduce
Use the tab key to reach the buttons in the public facing slides, check where the focus is (in the code, you can use document.activeElement in the browser developer tools console

Desktop (please complete the following information):

• OS: Windows 11
• Browser: Chrome (also Edge and Firefox, it is crossbrowser issue)
• Slidev version: 0.47.4

Possible fix
I will have to dig into code to verify this, but it seems that tailwind focus-within (https://tailwindcss.com/docs/hover-focus-and-other-states#pseudo-classes) could be used to set opacity to 100 (current classes used are opacity-0 hover:opacity-100 opacity-100).