Add RTC time persistence to flash

Save last known unix timestamp to /rtc_save on shutdown and restore it
on startup, so the clock shows correct time immediately after restart
without waiting for GPS fix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/MyMesh.h

@@ -165,6 +165,7 @@ protected:
 
 public:
   void savePrefs() { _store->savePrefs(_prefs, sensors.node_lat, sensors.node_lon); }
+  void saveRTCTime() { _store->saveRTCTime(); }
 
 #if ENV_INCLUDE_GPS == 1
   void applyGpsPrefs() {