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/services/fit_parser.py

@@ -34,6 +34,16 @@ def _safe_float(val) -> Optional[float]:
         return None
 
 
+def _sanitize_speed(val, dist_m=None, dur_s=None) -> Optional[float]:
+    """Reject the FIT invalid sentinel (0xFFFF/1000 = 65.535 m/s) and fall back to dist/dur."""
+    fv = _safe_float(val)
+    if fv is None or fv >= 65.0:
+        if dist_m and dur_s and float(dur_s) > 0:
+            return float(dist_m) / float(dur_s)
+        return None
+    return fv
+
+
 def _bounding_box(coords):
     if not coords:
         return None
@@ -180,15 +190,19 @@ def parse_fit_file(filepath: str) -> dict:
     normalized_laps = []
     for i, lap in enumerate(laps):
         ls = _to_dt(get(lap, "startTime", "start_time"))
+        lap_dist = _safe_float(get(lap, "totalDistance", "total_distance"))
+        lap_dur = _safe_float(get(lap, "totalElapsedTime", "total_elapsed_time"))
         normalized_laps.append({
             "lap_number": i + 1,
             "start_time": ls.isoformat() if ls else None,
-            "duration_s": _safe_float(get(lap, "totalElapsedTime", "total_elapsed_time")),
-            "distance_m": _safe_float(get(lap, "totalDistance", "total_distance")),
+            "duration_s": lap_dur,
+            "distance_m": lap_dist,
             "avg_heart_rate": _safe_float(get(lap, "avgHeartRate", "avg_heart_rate")),
             "avg_cadence": _safe_float(get(lap, "avgCadence", "avg_cadence")),
-            "avg_speed_ms": _safe_float(get(lap, "avgSpeed", "avg_speed",
-                                            "enhancedAvgSpeed", "enhanced_avg_speed")),
+            "avg_speed_ms": _sanitize_speed(
+                get(lap, "avgSpeed", "avg_speed", "enhancedAvgSpeed", "enhanced_avg_speed"),
+                dist_m=lap_dist, dur_s=lap_dur,
+            ),
             "avg_power": _safe_float(get(lap, "avgPower", "avg_power")),
         })
 
@@ -209,8 +223,11 @@ def parse_fit_file(filepath: str) -> dict:
         "avg_cadence": _safe_float(get(session_data, "avgCadence", "avg_cadence")),
         "avg_power": _safe_float(get(session_data, "avgPower", "avg_power")),
         "normalized_power": _safe_float(get(session_data, "normalizedPower", "normalized_power")),
-        "avg_speed_ms": _safe_float(get(session_data, "avgSpeed", "avg_speed",
-                                        "enhancedAvgSpeed", "enhanced_avg_speed")),
+        "avg_speed_ms": _sanitize_speed(
+            get(session_data, "avgSpeed", "avg_speed", "enhancedAvgSpeed", "enhanced_avg_speed"),
+            dist_m=_safe_float(get(session_data, "totalDistance", "total_distance")),
+            dur_s=_safe_float(get(session_data, "totalElapsedTime", "total_elapsed_time")),
+        ),
         "max_speed_ms": _safe_float(get(session_data, "maxSpeed", "max_speed",
                                         "enhancedMaxSpeed", "enhanced_max_speed")),
         "avg_temperature_c": _safe_float(get(session_data, "avgTemperature", "avg_temperature")),