refactor(trail): route gpsHasFix through ownPos/currentLocation

The last stray getLocationProvider/isValid duplicate (with its own
ENV_INCLUDE_GPS guard). Fold it into ownPos so every GPS lookup in the
nav screens goes through UITask::currentLocation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/TrailScreen.h

@@ -218,15 +218,8 @@ public:
     return false;
   }
 
-  // True when the global SensorManager has a usable GPS fix.
+  // True when there's a usable GPS fix (same source as ownPos).
-  static bool gpsHasFix() {
+  bool gpsHasFix() const { int32_t lat, lon; return ownPos(lat, lon); }
-#if ENV_INCLUDE_GPS == 1
-    LocationProvider* loc = sensors.getLocationProvider();
-    return loc && loc->isValid();
-#else
-    return false;
-#endif
-  }
 
 private:
   void handleToggle() {