Add athlete profile pages for other users

Problem

profile.stx is hardwired to the current user (currentUser()/userStats()); there's no /athlete/{id} route and feed author names are plain text, not links. You can't view anyone else's profile.

Acceptance

• /athlete/{id} view backed by a public-profile endpoint (stats, recent activities, territories, achievements).
• Link author names (feed, leaderboard, conquests) to profiles.

Done (commit f720b1f). AthleteShowAction (GET /api/users/{id}) returns identity + aggregated stats (activities, distance/elevation, territories owned/area/conquered) + follower/following counts + recent activities. New /athlete/{id} page with stats, a Follow button, and recent activities. Feed author names and leaderboard rows now link to athlete profiles. Verified via harness.