- Integrate the Zephyr Settings subsystem to persist Modbus parameters.
- Use NVS (Non-Volatile Storage) as the backend with a dedicated flash partition.
- Modbus baudrate and slave ID are now loaded at startup.
- Changes made via the shell are saved to flash and survive a reboot.
- Add a 'reset' command to the shell for easier testing.
- Fix all compiler and devicetree warnings for a clean build.
