fix: code review corrections in DualSerialInterface and NearbyScreen

- DualSerialInterface: guard writeFrame/isWriteBusy/isBLEConnected with
  _ble_enabled to avoid BLE-after-disable race
- NearbyScreen: initialize all fields in constructor; promote D_* layout
  constants to class scope; clamp _dscroll on result count change;
  replace hardcoded 2 with D_VISIBLE in scroll logic; allow re-scan
  from detail view via KEY_CONTEXT_MENU/KEY_ENTER
- variant.cpp: fix inconsistent indentation in initVariant()

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

variants/wio-tracker-l1/variant.cpp

@@ -78,5 +78,4 @@ void initVariant() {
     // set buzzer pin as output and set it low
     pinMode(12, OUTPUT);
     digitalWrite(12, LOW);
-    pinMode(12, OUTPUT);
 }