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

The issue lists several cleanup tasks for Svelte 5 migration. Start by examining edit/NeighbourhoodMode.svelte to remove duplicate renderNeighbourhood() calls. Look for verbose $props interfaces across components to simplify. Check NeighbourhoodBoundarySummary and Modals for HTML whitespace formatting issues with prettier. Each task requires familiarity with the codebase's Svelte components and build setup.

Written by the indexing model from the issue text.