- Replace `{{Replies}}`/`{{Reactions}}` stat items in digest.mjml with
a single `{{RelayUrl}}` item showing the posting relay domain
- src/digest.ts: add `RelayUrl` to template variables, derive from
existing `relayByEventId` map; strip unused `spec`, `getParentId`,
`repliesByParentId`, and `context` destructure
- test/digest-reply-stats.test.ts: update assertion to check
`parentEntry.RelayUrl` instead of `Replies`/`Reactions`
- script/render-preview.mjs: update sample data to use `RelayUrl`
The buildParameters function returns events sorted newest-first
(sortBy created_at, slice(0,100)), and the mjml section is already
labeled 'Latest Activity'. The object keys Popular/HasPopular no
longer reflect the semantics, so rename them to Latest/HasLatest.
