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/UITask.h

@@ -152,6 +152,10 @@ public:
   // Current course over ground in degrees (0..359), or false if not enough
   // recent movement to derive a stable heading. Independent of trail logging.
   bool currentCourse(int& deg_out) const;
+  // Current GPS position (1e6-scaled degrees), false when there's no usable
+  // fix. Single source of truth for "where am I", shared by the nav / compass
+  // / map screens so the LocationProvider lookup isn't duplicated per screen.
+  bool currentLocation(int32_t& lat, int32_t& lon) const;
   void playMelody(const char* melody);
   void stopMelody();
   bool isMelodyPlaying();