fix: DEM elevation overcounting and add hysteresis-only recalculation button

- dem.py: apply 45s median filter before hysteresis to suppress SRTM
  tile-boundary steps that were accumulating through the 5m threshold;
  raise DEM hysteresis threshold from 5m to 10m
- dem.py: back up elevation_m as elevation_m_original in timeseries
  before the first DEM overwrite, so original sensor data is preserved
- dem.py: add recalculate_elevation_hysteresis() — recomputes gain/loss
  from original recorded elevation (reads elevation_m_original if a DEM
  run already replaced elevation_m) using source-aware thresholds
  (5m barometric, 10m GPS/unknown); does not touch the elevation array
- edit/server.py, serve/server.py: split /recalculate-elevation into
  two endpoints: /recalculate-elevation/dem and
  /recalculate-elevation/hysteresis
- EditDrawer.svelte: replace single DEM button with two side-by-side
  buttons — "Recalculate (hysteresis)" (fast, offline) and
  "Recalculate (DEM)" (SRTM lookup)

bincio/serve/server.py

@@ -1267,8 +1267,8 @@ async def post_activity(
     return JSONResponse({"ok": True})
 
 
-@app.post("/api/activity/{activity_id}/recalculate-elevation")
-async def recalculate_elevation_endpoint(
+@app.post("/api/activity/{activity_id}/recalculate-elevation/dem")
+async def recalculate_elevation_dem_endpoint(
     activity_id: str,
     bincio_session: Optional[str] = Cookie(default=None),
 ) -> JSONResponse:
@@ -1296,6 +1296,30 @@ async def recalculate_elevation_endpoint(
         raise HTTPException(422, str(e))
 
 
+@app.post("/api/activity/{activity_id}/recalculate-elevation/hysteresis")
+async def recalculate_elevation_hysteresis_endpoint(
+    activity_id: str,
+    bincio_session: Optional[str] = Cookie(default=None),
+) -> JSONResponse:
+    """Recompute gain/loss from original recorded elevation using source-aware hysteresis."""
+    user = _require_user(bincio_session)
+    _check_id(activity_id)
+    dd = _get_data_dir() / user.handle
+    if not (dd / "activities" / f"{activity_id}.json").exists():
+        raise HTTPException(404, "Activity not found")
+    try:
+        from bincio.extract.dem import recalculate_elevation_hysteresis
+        from bincio.render.merge import merge_one
+        result = recalculate_elevation_hysteresis(dd, activity_id)
+        merge_one(dd, activity_id)
+        _trigger_rebuild(user.handle)
+        return JSONResponse(result)
+    except FileNotFoundError as e:
+        raise HTTPException(404, str(e))
+    except ValueError as e:
+        raise HTTPException(422, str(e))
+
+
 @app.delete("/api/activity/{activity_id}", response_model=GenericResponse)
 async def delete_activity(
     activity_id: str,