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)

README.md

@@ -250,7 +250,8 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
 ```
 
 ## OLED Behavior
-- Sender: OLED stays **ON for 10 seconds** on each wake, then powers down for sleep.
+- Sender: OLED stays on for `OLED_AUTO_OFF_MS` after boot or last activity.
+- Activity is detected while `PIN_OLED_CTRL` is held high, or on the high→low edge when the control is released.
 - Receiver: OLED is always on (no auto-off).
 - Pages rotate every 4s.
 
@@ -260,6 +261,7 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
   - 4.2 V = 100%
 - Uses deep sleep between cycles (`SENDER_WAKE_INTERVAL_SEC`).
 - Sender CPU is throttled to 80 MHz and LoRa RX is only enabled in short windows (ACK wait or time-sync).
+- Battery sampling uses a single ADC read and updates at most once per `BATTERY_SAMPLE_INTERVAL_MS` (default 60s).
 
 ## Web UI
 - AP SSID: `DD3-Bridge-<short_id>` (prefix configurable)
@@ -311,9 +313,10 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
 
 ## Config Knobs
 Key timing settings in `include/config.h`:
-- `METER_SAMPLE_INTERVAL_MS`
-- `METER_SEND_INTERVAL_MS`
-- `BATCH_ACK_TIMEOUT_MS`
+ - `METER_SAMPLE_INTERVAL_MS`
+ - `METER_SEND_INTERVAL_MS`
+ - `BATTERY_SAMPLE_INTERVAL_MS`
+ - `BATCH_ACK_TIMEOUT_MS`
 - `BATCH_MAX_RETRIES`
  - `BATCH_QUEUE_DEPTH`
  - `BATCH_RETRY_POLICY` (keep or drop on retry exhaustion)