feat(settings): Implement persistent Modbus configuration

- 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.

software/apps/slave_node/prj.conf

@@ -12,6 +12,16 @@ CONFIG_USE_SEGGER_RTT=y
 # Enable Shell
 CONFIG_SHELL=y
 CONFIG_SHELL_BACKEND_RTT=y
+CONFIG_REBOOT=y
+
+# Enable Settings Subsystem
+CONFIG_SETTINGS=y
+CONFIG_SETTINGS_NVS=y
+CONFIG_NVS=y
+CONFIG_FLASH=y
+CONFIG_FLASH_MAP=y
+CONFIG_FLASH_PAGE_LAYOUT=y
+CONFIG_SETTINGS_LOG_LEVEL_DBG=y
 
 # Config modbus
 CONFIG_UART_INTERRUPT_DRIVEN=y