Add fallback logic if any layer fails to load

[itspangler]

Per our convo in Notion, we should add some kind of global fallback logic in the event a layer fails to load. It should:

• update menu
• update bibliographic details
• possibly have an alert so that the user knows that they are seeing something other than what was expected

We could branch the logic such that 1) if a modern base layer (MapTiler, satellite) fails to fetch, default to OSM; and 2) if an atlas layer fails to fetch, cascade through the list of other available layers, then fall back to OSM—or, we could just have everything fall back to OSM.

Ideally we can incorporate this before pushing Svelte 5 to production.