Add segments, YTD stats, route matching fixes, body battery layout, pace fix

- Segments page: new /segments route with auto-generate (1km splits, turn
  detection, hill detection), manual segment creation, per-segment performance
  times across matched activities; fixed auth on existing segment endpoints
- YTD distance: new /activities/stats/ytd endpoint; Dashboard replaces
  'Total distance' with 'Running this year' + 'Cycling this year'; Activities
  page shows YTD stats row
- Weekly chart click: clicking a Dashboard bar navigates to Activities filtered
  to that week; Activities reads from/to query params with dismissable chip
- Route matching: add ±2.5% distance gate + 3% relative DTW threshold
  (was flat 80m); tighten candidate pre-filter from 80/120% to 95/105%
- Body battery layout: HR chart and body battery now side-by-side at same
  height on large screens instead of stacked full-width
- Pace display fix: MetricTimeline clamps GPS speed outliers before computing
  Y-axis domain; tick formatter guards against v<=0 or v>25 m/s

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/activities.py

@@ -75,6 +75,30 @@ class LapOut(BaseModel):
         from_attributes = True
 
 
+@router.get("/stats/ytd")
+async def ytd_stats(
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Return year-to-date distance totals grouped by sport type."""
+    from datetime import date, timezone
+    year_start = datetime(date.today().year, 1, 1, tzinfo=timezone.utc)
+    result = await db.execute(
+        select(Activity.sport_type, func.sum(Activity.distance_m).label("total_m"))
+        .where(Activity.user_id == current_user.id, Activity.start_time >= year_start)
+        .group_by(Activity.sport_type)
+    )
+    rows = result.all()
+    totals = {r.sport_type: (r.total_m or 0) / 1000 for r in rows}
+    return {
+        "running_km": round(totals.get("running", 0), 2),
+        "cycling_km": round(totals.get("cycling", 0), 2),
+        "hiking_km": round(totals.get("hiking", 0), 2),
+        "walking_km": round(totals.get("walking", 0), 2),
+        "total_km": round(sum(totals.values()), 2),
+    }
+
+
 @router.get("/", response_model=List[ActivitySummary])
 async def list_activities(
     page: int = Query(1, ge=1),