Fix OLED autosleep timing and battery sampling cadence

- Track last OLED activity to avoid double timeout; keep power gating on transitions
- Copy TZ before setenv() in timegm_fallback to avoid invalid pointer reuse
- Add BATTERY_SAMPLE_INTERVAL_MS and only refresh cache at batch start when due
- Keep battery sampling to a single ADC read (Arduino core lacks explicit ADC power gating)

src/main.cpp

@@ -152,6 +152,10 @@ static void update_battery_cache() {
   g_last_battery_ms = millis();
 }
 
+static bool battery_sample_due(uint32_t now_ms) {
+  return g_last_battery_ms == 0 || now_ms - g_last_battery_ms >= BATTERY_SAMPLE_INTERVAL_MS;
+}
+
 static bool sender_timesync_window_due() {
   uint32_t interval_sec = SENDER_TIMESYNC_CHECK_SEC_FAST;
   if (time_is_synced() && time_rtc_present()) {
@@ -742,7 +746,7 @@ static void sender_loop() {
       note_fault(g_sender_faults, g_sender_last_error, g_sender_last_error_utc, g_sender_last_error_ms, FaultType::MeterRead);
       display_set_last_error(g_sender_last_error, g_sender_last_error_utc, g_sender_last_error_ms);
     }
-    if (g_build_count == 0) {
+    if (g_build_count == 0 && battery_sample_due(now_ms)) {
       update_battery_cache();
     }
     data.battery_voltage_v = g_last_battery_voltage_v;