refactor(nav): single GPS-position accessor + reset COG window after a gap

UITask::currentLocation() becomes the one place that reads the
LocationProvider fix. TrailScreen::ownPos, CompassScreen::gpsValid and
NearbyScreen::refresh now delegate to it instead of each duplicating the
getLocationProvider/isValid dance (and the ENV_INCLUDE_GPS guard).

pushCogFix now restarts the course-over-ground ring when the gap since the
previous fix exceeds 15 s: after losing GPS for a while, a window spanning
the stale old fixes would compute a bogus "teleport" heading. The
last-good heading is kept for display continuity.

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

examples/companion_radio/ui-new/CompassScreen.h

@@ -13,14 +13,7 @@
 class CompassScreen : public UIScreen {
   UITask* _task;
 
-  static bool gpsValid() {
-#if ENV_INCLUDE_GPS == 1
-    LocationProvider* loc = sensors.getLocationProvider();
-    return loc && loc->isValid();
-#else
-    return false;
-#endif
-  }
+  bool gpsValid() const { int32_t lat, lon; return _task->currentLocation(lat, lon); }
 
   // Midpoint circle (DisplayDriver has no circle primitive).
   static void drawCircle(DisplayDriver& d, int cx, int cy, int r) {