feat(modbus): Add supply voltage register and display in tool

This commit introduces a new Modbus input register for the system's supply voltage.

- The `modbus-registers.de.md` documentation is updated to include the `SUPPLY_VOLTAGE_MV` register at address `0x00F5` within the system block.
- The `modbus_server.h` header defines the new register.
- The `modbus_server.c` implementation provides a fixed value (12300 mV) for this register.
- The `modbus_tool.py` script is updated to read and display this new supply voltage value in the UI.

This lays the groundwork for integrating actual voltage measurements in the future.

software/lib/modbus_server/modbus_server.c

@@ -128,6 +128,9 @@ static int input_reg_rd(uint16_t addr, uint16_t *reg)
 	case REG_INPUT_UPTIME_SECONDS_HIGH:
 		*reg = (uint16_t)(uptime_s >> 16);
 		break;
+	case REG_INPUT_SUPPLY_VOLTAGE_MV:
+		*reg = 12300;
+		break;
 	case REG_INPUT_FWU_LAST_CHUNK_CRC:
 		*reg = fwu_get_last_chunk_crc();
 		break;