Add battery min/max voltage parameter support

variants/lilygo_tbeam_1w/platformio.ini

@@ -31,6 +31,10 @@ build_flags =
   ; TX power: 22dBm to SX1262, PA module adds ~10dB for 32dBm total
   -D LORA_TX_POWER=22
 
+  ; Battery - 2S 7.4V LiPo (6.0V min, 8.4V max)
+  -D BATT_MIN_MILLIVOLTS=6000
+  -D BATT_MAX_MILLIVOLTS=8400
+
   ; Display - SH1106 OLED at 0x3C
   -D DISPLAY_CLASS=SH1106Display