The tricky part here was that `returnPartialData` seems to behave differently during SSR. On the page itself, this seems to cause us to always get back at least an empty object, but in SSR we can sometimes get null—which means that a LOT of code that expects the item object to exist while in loading state gets thrown off.
To keep this situation maximally clear, I added a bunch of null handling with `?.` to `ItemPageLayout`. An alternative would have been to check for null and put in an empty object if not, but this feels more resilient and more true to the situation.
The search bar here is a bit tricky, but is pretty straightforwardly adapted from how we did the layouts in App.js. Fingers crossed that it works as smoothly as expected when the search page is migrated too! (Right now typing in there is all messy because it hops over to the fallback route and does its whole separate thing.)
