feat(modbus): Implement persistent and improved reconfiguration for Modbus server

This commit enhances the Modbus server's configuration handling by:

- Loading saved baudrate and unit ID settings during initialization, ensuring persistence across reboots.
- Providing improved feedback during `modbus_reconfigure`, including logging for successful changes and informing the user when a device restart is required for changes to take effect.
- Saving new configuration settings even if immediate reinitialization fails, allowing them to be applied on the next boot.