Fix pace sentinel, route map thumbnails, tiled segments, health/dashboard layout

- Pace: FIT 0xFFFF sentinel (65.535 m/s) was stored as avg_speed_ms on every
  activity and lap; add _sanitize_speed() to parser falling back to dist/dur,
  plus a startup SQL migration that fixed 120 activities and 688 laps in-place
- Records: remove swimming from Distance PRs; Route Records rows are clickable
  (navigate to activity), View button removed, small SVG route map per row;
  Segment Records uses same tiled route-card layout as Segments page
- Segments: replace route dropdown with responsive tile grid showing SVG map
  thumbnails; selecting a tile reveals the segment management panel below
- RouteMiniMap: new pure-SVG component (no Leaflet) for route thumbnails,
  decodes polyline and normalises coords into a fixed viewBox
- Health: rename "Avg Heart Rate (day)" → "Heart Rate"; weight chart now
  filters to non-null rows and enables connectNulls + dots for sparse data
- Dashboard: 4-col layout at lg breakpoint so Body Battery sits between weekly
  chart and Health Today; Body Battery card gains a 24-hr sparkline from the
  values[] already present in the health summary response

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

backend/app/api/records.py

@@ -58,6 +58,7 @@ async def route_records(
               nr.name     AS route_name,
               nr.sport_type,
               nr.distance_m,
+              nr.reference_polyline,
               a.id        AS activity_id,
               a.name      AS activity_name,
               a.duration_s,