Batch 1: dashboard, maps, segments rewrite, health, sync UX

Fixes:
- Dashboard: featured most-recent activity card with map + stats
- Maps default to Street; preferCanvas + larger tile buffer for smoother pan/zoom
- Running cadence as colour-banded dots + 165 spm guide line
- Routes: inline row expansion, rename (PATCH /routes/{id}), podium + deltas, tiled map
- Records: remove reversed pace Y-axis
- Profile: remove resting HR; add goal weight
- Health: snapshot weight carry-forward; VO2 trend axis 30-70;
  weight goal line + kg/st-lb toggle + axis max; sleep 8h/avg lines
- Garmin sync progress moved to global store with persistent floating bar

Features:
- Speed-coloured activity route (default) with Speed/Solid toggle
- GPS-geometry segments: draw on map, match across all activities,
  1st/2nd/3rd leaderboard + podium badges (replaces old distance segments)
- Lap bests: best time per lap across a route + delta column
- Body Battery: highlight activity time windows

Schema: users.goal_weight_kg ALTER; new segments/segment_efforts tables.
Removes RouteSegment, the Segments page, and segment-bests endpoints.

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

backend/app/api/activities.py

@@ -195,6 +195,37 @@ async def get_laps(
     return result.scalars().all()
 
 
+@router.get("/{activity_id}/lap-bests")
+async def get_lap_bests(
+    activity_id: int,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Best (fastest) time per lap number across all activities on the same route."""
+    act = (await db.execute(
+        select(Activity).where(
+            Activity.id == activity_id,
+            Activity.user_id == current_user.id,
+        )
+    )).scalar_one_or_none()
+    if not act:
+        raise HTTPException(status_code=404, detail="Activity not found")
+    if not act.named_route_id:
+        return {}
+
+    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,
+            ActivityLap.duration_s.isnot(None),
+        )
+        .group_by(ActivityLap.lap_number)
+    )).all()
+    return {str(lap_number): best for lap_number, best in rows}
+
+
 @router.patch("/{activity_id}/name")
 async def rename_activity(
     activity_id: int,