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/rtc_ds3231.cpp

@@ -1,6 +1,7 @@
 #include "rtc_ds3231.h"
 #include "config.h"
 #include <Wire.h>
+#include <string>
 #include <time.h>
 
 static constexpr uint8_t DS3231_ADDR = 0x68;
@@ -17,12 +18,14 @@ static time_t timegm_fallback(struct tm *tm_utc) {
   if (!tm_utc) {
     return static_cast<time_t>(-1);
   }
-  char *old_tz = getenv("TZ");
+  const char *old_tz = getenv("TZ");
+  // getenv() may return a pointer into mutable storage that becomes invalid after setenv().
+  std::string old_tz_copy = old_tz ? old_tz : "";
   setenv("TZ", "UTC0", 1);
   tzset();
   time_t t = mktime(tm_utc);
-  if (old_tz) {
-    setenv("TZ", old_tz, 1);
+  if (!old_tz_copy.empty()) {
+    setenv("TZ", old_tz_copy.c_str(), 1);
   } else {
     unsetenv("TZ");
   }