adding save and battery voltage reading

main/system.h

@@ -58,6 +58,13 @@ typedef struct SystemState_s
     // Swap L/R audio channels
     bool swapLR;
 
+    // Volume setting (0-100)
+    int volume;
+
+    // Battery monitoring
+    int batteryVoltage_mv;
+    int batteryPercentage;
+
 } SystemState_t;
 
 
@@ -93,6 +100,11 @@ float system_getAngle(void);
 void system_setChargeStatus(bool charging);
 bool system_getChargeStatus(void);
 
+void system_setBatteryVoltage(int voltage_mv);
+int system_getBatteryVoltage(void);
+void system_setBatteryPercentage(int percentage);
+int system_getBatteryPercentage(void);
+
 void system_setSwapLR(bool swap);
 bool system_getSwapLR(void);
 void system_toggleSwapLR(void);
@@ -118,6 +130,10 @@ int system_getBtDeviceIndex(void);
 // Volume control functions
 void system_requestVolumeUp(void);
 void system_requestVolumeDown(void);
+void system_setVolume(int volume);
+int system_getVolume(void);
+esp_err_t system_saveVolume(void);
+esp_err_t system_loadVolume(void);
 
 // NVS Service
 typedef enum {