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/main.cpp

@@ -211,6 +211,7 @@ void setup() {
   #error "need to define filesystem"
 #endif
 
+  store.restoreRTCTime();
   sensors.begin();
 
 #if ENV_INCLUDE_GPS == 1