Fix content layout and transitions

• which elements can be transitioned to/from?
• is it worth to exit astro's transition (which has a polyfill for Firefox)? if not, is sliding the only option? then use slide on the main grid and persist on header and footer, viole done.
• add prefetch so that pages load instantaneously: https://docs.astro.build/en/guides/prefetch

Answers: see below

Implementation decisions:

• Use Astro's polyfill with alpha transition
• Persist header and footer
• Wait for a better future to implement morphing