Fix 1970 date after crash/watchdog/brownout reset on ESP32

Currently, time is only set to ~2024 when it powers on cleanly. When
time is lost, this results in time dropping to 1970. Save time as backup
to RTC slow memory. Should fix time staying accurate during brownouts
etc.

src/helpers/ESP32Board.h

@@ -197,19 +197,30 @@ public:
   }
 };
 
+static RTC_NOINIT_ATTR uint32_t _rtc_backup_time;
+static RTC_NOINIT_ATTR uint32_t _rtc_backup_magic;
+#define RTC_BACKUP_MAGIC  0xAA55CC33
+#define RTC_TIME_MIN      1772323200  // 1 Mar 2026
+
 class ESP32RTCClock : public mesh::RTCClock {
 public:
   ESP32RTCClock() { }
   void begin() {
     esp_reset_reason_t reason = esp_reset_reason();
-    if (reason == ESP_RST_POWERON) {
-      // start with some date/time in the recent past
-      struct timeval tv;
-      tv.tv_sec = 1715770351;  // 15 May 2024, 8:50pm
+    if (reason == ESP_RST_DEEPSLEEP) {
+      return;  // ESP-IDF preserves system time across deep sleep
+    }
+    // All other resets (power-on, crash, WDT, brownout) lose system time.
+    // Restore from RTC backup if valid, otherwise use hardcoded seed.
+    struct timeval tv;
+    if (_rtc_backup_magic == RTC_BACKUP_MAGIC && _rtc_backup_time > RTC_TIME_MIN) {
+      tv.tv_sec = _rtc_backup_time;
+    } else {
+      tv.tv_sec = 1772323200;  // 1 Mar 2026
+    }
     tv.tv_usec = 0;
     settimeofday(&tv, NULL);
   }
-  }
   uint32_t getCurrentTime() override {
     time_t _now;
     time(&_now);
@@ -220,6 +231,16 @@ public:
     tv.tv_sec = time;
     tv.tv_usec = 0;
     settimeofday(&tv, NULL);
+    _rtc_backup_time = time;
+    _rtc_backup_magic = RTC_BACKUP_MAGIC;
+  }
+  void tick() override {
+    time_t now;
+    time(&now);
+    if (now > RTC_TIME_MIN && (uint32_t)now != _rtc_backup_time) {
+      _rtc_backup_time = (uint32_t)now;
+      _rtc_backup_magic = RTC_BACKUP_MAGIC;
+    }
   }
 };