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/ui-new/UITask.cpp

@@ -986,6 +986,7 @@ void UITask::setCurrScreen(UIScreen* c) {
   hardware-agnostic pre-shutdown activity should be done here
 */
 void UITask::shutdown(bool restart){
+  the_mesh.saveRTCTime();
 
   #ifdef PIN_BUZZER
   /* note: we have a choice here -