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/render/cli.py

@@ -418,6 +418,40 @@ def _backfill_vam_summary(data: Path, handle: str | None = None) -> None:
         console.print(f"  [cyan]{user_dir.name}[/cyan]: {updated} summary(ies) updated")
 
 
+def _backfill_speed(data: Path, handle: str | None = None) -> None:
+    """Compute GPS-derived speed for timeseries files where speed_kmh is all null.
+
+    Reads each *.timeseries.json, fills speed_kmh from haversine distances when
+    the device did not record per-second speed, and writes the file back.
+    """
+    import json
+    from bincio.extract.timeseries import _gps_speed_kmh
+
+    targets = [data / handle] if handle else _user_dirs(data)
+    for user_dir in targets:
+        acts_dir = user_dir / "activities"
+        if not acts_dir.exists():
+            continue
+        updated = 0
+        for ts_path in sorted(acts_dir.glob("*.timeseries.json")):
+            try:
+                ts = json.loads(ts_path.read_text(encoding="utf-8"))
+            except Exception:
+                continue
+            spd = ts.get("speed_kmh", [])
+            if not spd or any(v is not None for v in spd):
+                continue  # already has speed data
+            lat_vals = ts.get("lat") or []
+            lon_vals = ts.get("lon") or []
+            t_vals   = ts.get("t") or []
+            if not lat_vals or not lon_vals or not t_vals:
+                continue
+            ts["speed_kmh"] = _gps_speed_kmh(lat_vals, lon_vals, t_vals)
+            ts_path.write_text(json.dumps(ts, indent=2, ensure_ascii=False), encoding="utf-8")
+            updated += 1
+        console.print(f"  [cyan]{user_dir.name}[/cyan]: {updated} timeseries updated with GPS speed")
+
+
 @click.command()
 @click.option("--config", "config_path", default=None,
               help="Path to extract_config.yaml (reads output.dir from it).")
@@ -444,6 +478,9 @@ def _backfill_vam_summary(data: Path, handle: str | None = None) -> None:
 @click.option("--backfill-vam-summary", "backfill_vam_summary", is_flag=True,
               help="Copy climbing_vam_mh from detail JSONs into index.json summaries "
                    "(run once after the VAM curve → summary migration).")
+@click.option("--backfill-speed", "backfill_speed", is_flag=True,
+              help="Compute GPS-derived speed for timeseries where the device didn't record "
+                   "per-second speed (run once to enable speed map coloring on older activities).")
 def render(
     config_path: Optional[str],
     data_dir: Optional[str],
@@ -456,6 +493,7 @@ def render(
     recompute_climbs: bool,
     recompute_elevation: bool,
     backfill_vam_summary: bool,
+    backfill_speed: bool,
 ) -> None:
     """Build (or serve) the BincioActivity static site from a BAS data store."""
 
@@ -477,6 +515,10 @@ def render(
         console.print("Backfilling climbing_vam_mh into summaries…")
         _backfill_vam_summary(data, handle=handle)
 
+    if backfill_speed:
+        console.print("Backfilling GPS-derived speed into timeseries…")
+        _backfill_speed(data, handle=handle)
+
     _merge_edits(data, handle=handle)
     _rebuild_athlete_json(data, handle=handle)
     _bake_tracks(data, handle=handle)