fix(companion): prompt to enable GPS when starting a trail with GPS off

Starting trail recording with GPS switched off used to call setActive(true)
immediately -- the session timer ran while the screen sat on "Waiting for
GPS fix" forever and recorded nothing, with no hint that GPS was the
problem. Choosing "Start tracking" now opens a "GPS is off" confirmation
(Enable GPS & start / Cancel); confirming enables GPS and starts the
session. Behaviour is unchanged when GPS is already on.

Gated on a new UITask::hasGPS() so the prompt only appears on boards that
expose a toggleable GPS, not where GPS is simply absent.

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

examples/companion_radio/ui-new/UITask.h

@@ -323,6 +323,7 @@ public:
   void cycleBuzzerMode();   // ON → OFF → Auto → ON
   int  getBuzzerMode(); // 0=ON, 1=OFF, 2=Auto
   bool getGPSState();
+  bool hasGPS();   // true if this board exposes a toggleable GPS (distinct from GPS being off)
   void toggleGPS();
   void applyBrightness();
   void setBrightnessLevel(uint8_t level);