Consider using Bootstrap

In https://github.com/a-b-street/svelte-utils/issues/4, I've started consolidating Svelte helpers in a few other projects. There are two particular ideas to apply here:

1) Switching to Svelte 5, to unblock #368. From my experience so far with 5, I'm not yet convinced going all-in on the new runes is worth it. But we should be able to keep svelte 4 style, but still run with 5 and use wuchale, I think.

2) I've switched from picocss to Bootstrap in a few projects, and it's... less bad than pico. At least more maintained and complete. https://a-b-street.github.io/severance_snape/ and https://dabreegster.github.io/road-bundler/ are two projects converted to using this. Is it worth trying to do the same here? @michaelkirk heroically worked around dozens of pico issues here, especially maplibre/pico interactions. I don't want to go through that kind of effort again just to fit in well with Bootstrap, since Bootstrap is... fine, but not incredible.

Both of these points relate to `svelte-utils` too. We can't upgrade usage of that in this repo until we do both of the above. I've made some (I think) nicer basemap pickers and right-click controls in those newer projects, but I haven't thought about if they'd fit in well here yet.