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.cpp

@@ -288,6 +288,26 @@ void DataStore::savePrefs(const NodePrefs& _prefs, double node_lat, double node_
   }
 }
 
+void DataStore::saveRTCTime() {
+  uint32_t t = _clock->getCurrentTime();
+  if (t < 1000000000UL) return;  // don't save if time not yet synced
+  File file = openWrite(_fs, "/rtc_save");
+  if (file) {
+    file.write((uint8_t *)&t, sizeof(t));
+    file.close();
+  }
+}
+
+void DataStore::restoreRTCTime() {
+  File file = openRead(_fs, "/rtc_save");
+  if (file) {
+    uint32_t t = 0;
+    file.read((uint8_t *)&t, sizeof(t));
+    file.close();
+    if (t > 1000000000UL) _clock->setCurrentTime(t);
+  }
+}
+
 void DataStore::loadContacts(DataStoreHost* host) {
 File file = openRead(_getContactsChannelsFS(), "/contacts3");
     if (file) {

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;};

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() {

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

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 -