Fix follow-ups: lap bests, segments, charts, dashboard health

- Lap bests: compare against OTHER activities on the route (exclude self),
  so single-activity routes no longer show every lap as "best"
- Segment create: POST to trailing-slash URL (was a 307 that dropped the body);
  surface errors in the UI
- PR splits: scale GPS distance stream to the activity's official distance so
  over-measured GPS no longer yields bogus split PRs
- Speed route colours: red->orange->green->blue->purple (slow->fast) with smooth
  interpolation + a Slow/Fast gradient key under the map
- Health body battery: snap activity highlight to the categorical axis;
  white tooltip text + % suffix
- Health weight: y-min = lowest weight - 20kg; st/lb hover shows total lbs too
- Health sleep: move 8h/avg reference labels into the right margin
- Dashboard: Health-today pulls latest non-null values (sleep score, VO2 max);
  body battery tile renders a condensed colour-graded intraday graph

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/activities.py

@@ -213,12 +213,15 @@ async def get_lap_bests(
     if not act.named_route_id:
         return {}
 
+    # Best per lap number across OTHER activities on the same route, so the
+    # comparison is meaningful (excluding this activity from its own benchmark).
     rows = (await db.execute(
         select(ActivityLap.lap_number, func.min(ActivityLap.duration_s))
         .join(Activity, Activity.id == ActivityLap.activity_id)
         .where(
             Activity.named_route_id == act.named_route_id,
             Activity.user_id == current_user.id,
+            Activity.id != activity_id,
             ActivityLap.duration_s.isnot(None),
         )
         .group_by(ActivityLap.lap_number)