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/boards/bluepill_f103rb.overlay

@@ -2,6 +2,7 @@
     chosen {
         zephyr,console = &rtt;
         zephyr,shell = &rtt;
+        zephyr,settings-partition = &storage_partition;
     };
 
     rtt: rtt {
@@ -12,6 +13,27 @@
         status = "okay";
     };
 };
+
+&flash0 {
+    partitions {
+        compatible = "fixed-partitions";
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        /* Application partition starts at the beginning of flash */
+        slot0_partition: partition@0 {
+            label = "image-0";
+            reg = <0x00000000 DT_SIZE_K(120)>;
+        };
+
+        /* Use the last 8K for settings */
+        storage_partition: partition@1E000 {
+            label = "storage";
+            reg = <0x0001E000 DT_SIZE_K(8)>;
+        };
+    };
+};
+
 &usart1 {
 	modbus0 {
 		compatible = "zephyr,modbus-serial";