feat: add 4Hz GPS update interval

- Switch LocationSettings to AndroidSettings with intervalDuration: 250ms.
- AndroidSettings provides the intervalDuration parameter that LocationSettings lacks.
- 250ms = 4Hz; higher rate is only achievable when GNSS is available (not indoors).

lib/data/services/gps_service.dart

@@ -15,9 +15,10 @@ class GpsService {
     LocationAccuracy accuracy = LocationAccuracy.high,
   }) {
     return Geolocator.getPositionStream(
-      locationSettings: LocationSettings(
+      locationSettings: AndroidSettings(
         accuracy: accuracy,
         distanceFilter: 0,
+        intervalDuration: Duration(milliseconds: 250),
       ),
     );
   }