fix: write timeseries directly when wheel's write_activity silently skips it

Diagnosed via on-device debug: build_timeseries produces 3271 points
correctly, but the installed wheel's write_activity has a silent
exception path that skips writing the timeseries file. The workaround
calls build_timeseries directly and writes the file if missing.

Also moves useTheme import to @/ThemeContext across all tab screens.

mobile/extraction/PyodideWebView.tsx

@@ -39,6 +39,13 @@ const PY_EXTRACT = [
   'ts_path      = outdir / "activities" / (act_id + ".timeseries.json")',
   'geojson_path = outdir / "activities" / (act_id + ".geojson")',
   '',
+  '# write_activity in the installed wheel silently skips timeseries — write it directly.',
+  'if not ts_path.exists():',
+  '    from bincio.extract.timeseries import build_timeseries as _bts',
+  '    _ts = _bts(activity.points, activity.started_at, "public")',
+  '    if _ts.get("t"):',
+  '        ts_path.write_text(json.dumps(_ts))',
+  '',
   'json.dumps({',
   '  "id": act_id,',
   '  "detail": json.loads(detail_path.read_text()),',