Activity detail: layout refactor + GPS-derived speed for map coloring

Layout: map + charts stacked left, stats panel (2-col) on the right.
Cadence moved to last stat. Charts sit directly below the map.

Speed coloring: most FIT files don't record per-second speed, leaving
timeseries speed_kmh all-null and the hover link dead. Fix: derive speed
from consecutive GPS coordinates (haversine + 5-pt moving average) when
the device didn't record it. Add --backfill-speed render flag to retrofit
existing timeseries files.

bincio/extract/timeseries.py

@@ -2,11 +2,45 @@
 the BAS timeseries object (parallel arrays)."""
 
 from datetime import datetime
+from math import atan2, cos, radians, sin, sqrt
 from typing import Optional
 
 from bincio.extract.models import DataPoint
 
 
+def _gps_speed_kmh(
+    lat_vals: list[Optional[float]],
+    lon_vals: list[Optional[float]],
+    ts_vals: list[int],
+) -> list[Optional[float]]:
+    """Compute speed (km/h) from consecutive GPS coordinates via haversine.
+    Applies a 5-point centred moving-average to reduce GPS noise.
+    """
+    n = len(ts_vals)
+    raw: list[Optional[float]] = [None] * n
+    for i in range(1, n):
+        la0, lo0 = lat_vals[i - 1], lon_vals[i - 1]
+        la1, lo1 = lat_vals[i], lon_vals[i]
+        dt = ts_vals[i] - ts_vals[i - 1]
+        if la0 is None or lo0 is None or la1 is None or lo1 is None or dt <= 0:
+            continue
+        dlat = radians(la1 - la0)
+        dlon = radians(lo1 - lo0)
+        a = sin(dlat / 2) ** 2 + cos(radians(la0)) * cos(radians(la1)) * sin(dlon / 2) ** 2
+        d_km = 2 * 6371.0 * atan2(sqrt(a), sqrt(1 - a))
+        raw[i] = d_km / dt * 3600.0
+
+    # 5-point centred moving average (skip None anchors)
+    half = 2
+    smoothed: list[Optional[float]] = [None] * n
+    for i in range(n):
+        vals = [raw[j] for j in range(max(0, i - half), min(n, i + half + 1)) if raw[j] is not None]
+        if vals:
+            smoothed[i] = round(sum(vals) / len(vals), 2)
+
+    return smoothed
+
+
 def build_timeseries(
     points: list[DataPoint],
     started_at: datetime,
@@ -40,6 +74,11 @@ def build_timeseries(
     lon_vals  = [round(p.lon, 7) if p.lon is not None else None for p in sampled] if include_gps else None
     ele_vals  = [round(p.elevation_m, 1) if p.elevation_m is not None else None for p in sampled]
     spd_vals  = [round(p.speed_kmh, 2) if p.speed_kmh is not None else None for p in sampled]
+
+    # Derive speed from GPS when the device didn't record per-second speed.
+    if include_gps and lat_vals and lon_vals and all(v is None for v in spd_vals):
+        spd_vals = _gps_speed_kmh(lat_vals, lon_vals, ts_vals)
+
     hr_vals   = [p.hr_bpm for p in sampled]
     cad_vals  = [p.cadence_rpm for p in sampled]
     pwr_vals  = [p.power_w for p in sampled]