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/src/shell_system.c

@@ -0,0 +1,12 @@
+#include <zephyr/shell/shell.h>
+#include <zephyr/sys/reboot.h>
+
+static int cmd_reset(const struct shell *sh, size_t argc, char **argv)
+{
+	shell_print(sh, "Rebooting system...");
+	k_sleep(K_MSEC(100)); // Allow the shell to print the message
+	sys_reboot(SYS_REBOOT_WARM);
+	return 0;
+}
+
+SHELL_CMD_REGISTER(reset, NULL, "Reboot the system", cmd_reset);