Shared URL state

It would be great if those parts of the UI could be shared via URL state
- Mode / Primary navigation
- Sidealks > QA (Dropdown selection)
- Map State (zoom, lat, lng) (not relying on the default hash param solution)

Possibly also
- About "page" open-state

Some more notes on this are in https://github.com/a-b-street/speedwalk/pull/40

The issue mentions sharing UI state via URL, including mode, sidebar QA dropdown, map zoom/lat/lng, and possibly the About page open state. Start by examining the existing routing and state management in the Svelte app, likely in src/routes or components handling navigation and map. Look at how map state is currently managed and consider integrating with Svelte's stores or a router like svelte-spa-router. Check PR #40 for additional context. Done means each specified UI element updates the URL and restores state on page load.

Written by the indexing model from the issue text.