Add battery level indicator and optimize display layout

- Add battery icon with percentage display on main screen
- Implement battery voltage reading via ADC pin 4
- Remove network ID from main display to reduce clutter
- Reorganize UI layout for better space utilization:
  * Move battery icon to prevent text overflow
  * Compact header to single device info line
  * Gain 10 pixels vertical space for device list
- Battery updates every 30 seconds to conserve power
- Network ID still visible on 2-second startup screen

src/config/config.h

@@ -7,6 +7,7 @@
 #define IO_TXD2 17
 #define I2C_SDA 39
 #define I2C_SCL 38
+#define BAT_PIN 4
 
 // UWB Configuration
 #ifndef UWB_INDEX
@@ -26,6 +27,7 @@
 #define DISPLAY_UPDATE_INTERVAL 500
 #define DEVICE_TIMEOUT 5000
 #define SLEEP_DURATION 5000
+#define BATTERY_UPDATE_INTERVAL 30000
 
 // Display Configuration
 #define SCREEN_WIDTH 128