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/main.py

@@ -106,6 +106,20 @@ async def init_db():
     except Exception as e:
         print(f"FK migration skipped: {e}")
 
+    # Fix avg_speed_ms stored as the FIT invalid sentinel (0xFFFF/1000 = 65.535 m/s)
+    try:
+        async with engine.begin() as conn:
+            await conn.execute(text(
+                "UPDATE activities SET avg_speed_ms = distance_m / duration_s "
+                "WHERE avg_speed_ms > 30 AND distance_m > 0 AND duration_s > 0"
+            ))
+            await conn.execute(text(
+                "UPDATE activity_laps SET avg_speed_ms = distance_m / duration_s "
+                "WHERE avg_speed_ms > 30 AND distance_m > 0 AND duration_s > 0"
+            ))
+    except Exception as e:
+        print(f"avg_speed_ms fix skipped: {e}")
+
     # Seed admin user (only if password is configured)
     if not settings.admin_password:
         print("ADMIN_PASSWORD not set - skipping admin user seed")