Svelte 5 cleanups

These are a long tail of non-blocking problems leftover from migrating to Svelte 5.

- [x] **Breaking**: drawing a freehand line of filters doesn't show the line consistently. It stops after so many points or moving too fast?
- [ ] In edit/NeighbourhoodMode.svelte, don't call `renderNeighbourhood()` twice at startup
- [ ] Reintroduce a `Popup` wrapper that plumbs props succinctly
- [ ] Clean up places with the verbose `$props` interfaces -- inline the declaration or maybe go Vue-style and do `let props: { ... } = $props`
- [ ] Try to fix prettier formatting again (htmlWhitespaceSensitivity) -- see NeighbourhoodBoundarySummary and Modals for bad problems