fix(handbook): check for a new build on every navigation

[nerdalytics]

Description

After #179 deployed, an open tab kept showing the previous build with v2001.0.0 in the version selector. The server was already correct. version.json returned the new build id, the new worker precached 417 paths that all return 200, and /v2001.0.1/introduction returned 200. The tab stayed stale because the layout asked for the build version once, on mount. GitHub Pages serves every file with max-age=600, so a check in the ten minutes after a deploy can receive the old version.json from the CDN, and no client-side navigation asked again. This PR moves the check to afterNavigate, so every navigation re-checks in the background while the page still renders from the worker's cache.

Changes Made

• handbook/src/routes/+layout.svelte calls updated.check() from afterNavigate instead of onMount. afterNavigate runs after the initial load and after every client-side navigation.
• The service worker is unchanged. It stays cache-first and keeps passing version.json through with cache: 'no-store'.
• The existing effect is unchanged. When updated.current turns true it calls registration.update(), polls until the new worker's cache is the only one, and reloads.
• The change adds no timer and no interval. If a check lands on a stale CDN copy, the reader's next navigation retries it.

Performance Impact

Each client-side navigation makes one extra request for _app/version.json, which is 27 bytes on the live site. Nothing changes for the core library.

Test Coverage

• npm run check in handbook/: svelte-check reported 374 files, 0 errors, 0 warnings.
• BASE_PATH=/beacon npm run build in handbook/ completed.
• To verify after this deploys, open the site, wait for the next handbook deploy, then click any sidebar link. The tab should reload on its own within a few seconds of the CDN serving the new version.json, without a manual reload.
• No automated test covers this. The handbook has no browser test setup.

Related Issues

This follows #179. The maintainer reported the stale tab on 2026-10-19.

Checklist

• Tests added/updated for changes
• Documentation updated
• All tests passing
• Code formatting checked
• Performance implications considered
• Breaking changes documented (if applicable)