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/DataStore.h

@@ -49,6 +49,8 @@ public:
   bool removeFile(FILESYSTEM* fs, const char* filename);
   uint32_t getStorageUsedKb() const;
   uint32_t getStorageTotalKb() const;
+  void saveRTCTime();
+  void restoreRTCTime();
 
 private:
   FILESYSTEM* _getContactsChannelsFS() const { if (_fsExtra) return _fsExtra; return _fs;};