Nav, metadata and sitemap for the new routes #416

Make the new things findable. /ui and the packs need nav entries, their own titles and descriptions, canonical URLs, share cards and sitemap entries.

pageMeta already does all of this for existing routes, so this is mostly wiring plus the nav design, which currently has five links and no room for two more without a rethink.

Done when: every new route appears in the sitemap, shares with its own card, and routeMetadata.test.ts covers it.