fix(trail): fall back to list body in 'trail show' when detail fetch fails

The description block in 'trail show' dropped the body entirely whenever the best-effort detail fetch failed or the trail had no number (the enrich block is number-gated), even though the list body was already in hand. Seed the description from found.Body so degraded paths still render something; a successful detail fetch supersedes it with the richer body_document text.

Also note the prod web-origin assumption on trailWebURL and rename TestFetchTrailDetail_* to match the fetchTrailDescription it exercises.