feat(wio-tracker-l1): energy optimizations

- CPU sleep: board.sleep(0) in main loop puts NRF52 into WFE/sd_app_evt_wait
  between iterations; wakes on any interrupt (radio, BLE, timer). Largest
  power saving of the three changes.

- VBAT_ENABLE gating: voltage divider now powered only during ADC read
  (HIGH for 10ms, then LOW). Previously stayed HIGH continuously.
  Fixed in both variant.cpp (init) and WioTrackerL1Board::getBattMilliVolts().

- LED off with display: PIN_LED is cleared when auto-off triggers and
  restored (handed back to userLedHandler) when display wakes. Avoids
  idle LED current draw during screen-off periods.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

variants/wio-tracker-l1/variant.cpp

@@ -67,9 +67,9 @@ void initVariant() {
     pinMode(PIN_QSPI_CS, OUTPUT);
     digitalWrite(PIN_QSPI_CS, HIGH);
 
-    // VBAT_ENABLE
+    // VBAT_ENABLE: keep LOW by default, WioTrackerL1Board::getBattMilliVolts() toggles it
     pinMode(VBAT_ENABLE, OUTPUT);
-    digitalWrite(VBAT_ENABLE, HIGH);
+    digitalWrite(VBAT_ENABLE, LOW);
 
     // set LED pin as output and set it low
     pinMode(PIN_LED, OUTPUT);