Athlete segments tab: link best time to activity; expandable effort list

- best_activity_id now included in segment_summary API response
- Best time is a direct link to the activity that produced it
- Clicking a row expands an inline effort list (lazy-loaded from
  /api/segments/{id}/efforts): date linked to activity, time, Δ vs PR
- Clicking again collapses; ▲/▼ chevron shows state

bincio/serve/server.py

@@ -2699,8 +2699,9 @@ async def user_segment_summary(handle: str) -> JSONResponse:
                     "sport":      seg.sport,
                     "distance_m": seg.distance_m,
                 },
-                "best_elapsed_s": best.elapsed_s,
-                "effort_count":   len(efforts),
+                "best_elapsed_s":    best.elapsed_s,
+                "best_activity_id":  best.activity_id,
+                "effort_count":      len(efforts),
             })
     result.sort(key=lambda x: x["segment"]["name"].lower())
     return JSONResponse(result)