onroad UI: GPS speed display, hidden when no fix

Speed indicator now uses gpsLocation speed (MPH) instead of carState
vEgo. Hides completely when no GPS fix is available. Added gpsLocation
to UI SubMaster subscriptions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/ui/qt/onroad.h

@@ -55,6 +55,7 @@ private:
   QVBoxLayout *main_layout;
   QPixmap dm_img;
   float speed;
+  bool has_gps_speed = false;
   QString speedUnit;
   float setSpeed;
   float speedLimit;