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>

examples/companion_radio/main.cpp

@@ -229,4 +229,5 @@ void loop() {
   ui_task.loop();
 #endif
   rtc_clock.tick();
+  board.sleep(0);  // CPU sleeps until next interrupt (radio, timer, BLE); nRF52 ignores the seconds param
 }