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/main.c

@@ -10,6 +10,7 @@
 #include <zephyr/drivers/gpio.h>
 #include <zephyr/modbus/modbus.h>
 #include <zephyr/usb/usb_device.h>
+#include <zephyr/settings/settings.h>
 
 #include <zephyr/logging/log.h>
 #include "modbus_bridge.h"
@@ -164,6 +165,35 @@ uint8_t modbus_get_unit_id(void)
 	return server_param.server.unit_id;
 }
 
+static int settings_modbus_load(const char *name, size_t len,
+                                settings_read_cb read_cb, void *cb_arg)
+{
+    const char *next;
+    int rc;
+
+    if (settings_name_steq(name, "baudrate", &next) && !next) {
+        rc = read_cb(cb_arg, &server_param.serial.baud, sizeof(server_param.serial.baud));
+        if (rc < 0) {
+            return rc;
+        }
+        LOG_INF("Loaded modbus/baudrate: %u", server_param.serial.baud);
+        return 0;
+    }
+
+    if (settings_name_steq(name, "unit_id", &next) && !next) {
+        rc = read_cb(cb_arg, &server_param.server.unit_id, sizeof(server_param.server.unit_id));
+        if (rc < 0) {
+            return rc;
+        }
+        LOG_INF("Loaded modbus/unit_id: %u", server_param.server.unit_id);
+        return 0;
+    }
+
+    return -ENOENT;
+}
+
+SETTINGS_STATIC_HANDLER_DEFINE(modbus, "modbus", NULL, settings_modbus_load, NULL, NULL);
+
 
 static int init_modbus_server(void)
 {
@@ -183,6 +213,16 @@ static int init_modbus_server(void)
 int main(void)
 {
 	LOG_INF("Starting APP");
+
+	if (settings_subsys_init()) {
+		LOG_ERR("Failed to initialize settings subsystem");
+	}
+
+	if (settings_load()) {
+		LOG_ERR("Failed to load settings");
+	}
+
+
 	if (init_modbus_server()) {
 		LOG_ERR("Modbus RTU server initialization failed");
 	}
@@ -193,4 +233,4 @@ int main(void)
 	}
 
 	return 0;
-}
+}