feat(refactor): Restructure project for improved modularity and clarity

This commit introduces a major refactoring of the project structure to align
with Zephyr's recommended multi-application and library organization.

Key changes include:
- Relocation of custom modules from 'software/modules/' to 'software/lib/'.
- Introduction of a central 'software/CMakeLists.txt' to manage application
  and library subdirectories.
- Creation of new Kconfig files for 'software/' and 'software/apps/slave_node/'
  to define project-wide and application-specific configurations.
- Removal of the 'gateway' and 'stm32g431_tests' applications.
- Removal of 'shell_modbus.c' and 'shell_system.c' from 'slave_node' application's
  direct source files, indicating a shift towards library-based shell commands.
- Updates to 'software/apps/slave_node/CMakeLists.txt', 'prj.conf', and
  'boards/bluepill_f103rb.conf' to reflect the new structure and dependencies.

software/lib/shell_modbus/shell_modbus.c

@@ -0,0 +1,119 @@
+#include <zephyr/shell/shell.h>
+#include <stdlib.h>
+#include <lib/modbus_server.h>
+#include <lib/valve.h>
+
+static int cmd_modbus_set_baud(const struct shell *sh, size_t argc, char **argv)
+{
+	if (argc != 2) {
+		shell_error(sh, "Usage: set_baud <baudrate>");
+		return -EINVAL;
+	}
+
+	uint32_t new_baud = (uint32_t)strtoul(argv[1], NULL, 10);
+	const uint32_t valid_baud_rates[] = {1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200};
+	bool is_valid = false;
+
+	for (int i = 0; i < ARRAY_SIZE(valid_baud_rates); i++) {
+		if (new_baud == valid_baud_rates[i]) {
+			is_valid = true;
+			break;
+		}
+	}
+
+	if (!is_valid) {
+		char error_msg[128];
+		int offset = snprintf(error_msg, sizeof(error_msg), "Invalid baudrate. Valid rates are: ");
+		for (int i = 0; i < ARRAY_SIZE(valid_baud_rates); i++) {
+			offset += snprintf(error_msg + offset, sizeof(error_msg) - offset, "%u ", valid_baud_rates[i]);
+		}
+		shell_error(sh, "%s", error_msg);
+		return -EINVAL;
+	}
+
+	if (modbus_reconfigure(new_baud, modbus_get_unit_id()) != 0) {
+		shell_error(sh, "Failed to apply new baudrate");
+	} else {
+		shell_print(sh, "Modbus baudrate set to: %u (and saved)", new_baud);
+	}
+
+	return 0;
+}
+
+static int cmd_modbus_set_id(const struct shell *sh, size_t argc, char **argv)
+{
+	if (argc != 2) {
+		shell_error(sh, "Usage: set_id <slave_id>");
+		return -EINVAL;
+	}
+
+	uint32_t new_id_u32 = (uint32_t)strtoul(argv[1], NULL, 10);
+	if (new_id_u32 == 0 || new_id_u32 > 247) {
+		shell_error(sh, "Invalid slave ID: %s. Must be between 1 and 247.", argv[1]);
+		return -EINVAL;
+	}
+	uint8_t new_id = (uint8_t)new_id_u32;
+
+	if (modbus_reconfigure(modbus_get_baudrate(), new_id) != 0) {
+		shell_error(sh, "Failed to apply new slave ID");
+	} else {
+		shell_print(sh, "Modbus slave ID set to: %u (and saved)", new_id);
+	}
+
+	return 0;
+}
+
+static int cmd_valve_set_open_time(const struct shell *sh, size_t argc, char **argv)
+{
+    if (argc != 2) {
+        shell_error(sh, "Usage: set_open_time <seconds>");
+        return -EINVAL;
+    }
+
+    uint16_t seconds = (uint16_t)strtoul(argv[1], NULL, 10);
+    valve_set_max_open_time(seconds);
+    shell_print(sh, "Max opening time set to: %u seconds (and saved)", seconds);
+
+    return 0;
+}
+
+static int cmd_valve_set_close_time(const struct shell *sh, size_t argc, char **argv)
+{
+    if (argc != 2) {
+        shell_error(sh, "Usage: set_close_time <seconds>");
+        return -EINVAL;
+    }
+
+    uint16_t seconds = (uint16_t)strtoul(argv[1], NULL, 10);
+    valve_set_max_close_time(seconds);
+    shell_print(sh, "Max closing time set to: %u seconds (and saved)", seconds);
+
+    return 0;
+}
+
+static int cmd_config_show(const struct shell *sh, size_t argc, char **argv)
+{
+	shell_print(sh, "Current Modbus Configuration:");
+	shell_print(sh, "  Baudrate: %u", modbus_get_baudrate());
+	shell_print(sh, "  Slave ID: %u", modbus_get_unit_id());
+    shell_print(sh, "Current Valve Configuration:");
+    shell_print(sh, "  Max Opening Time: %u s", valve_get_max_open_time());
+    shell_print(sh, "  Max Closing Time: %u s", valve_get_max_close_time());
+	return 0;
+}
+
+SHELL_STATIC_SUBCMD_SET_CREATE(sub_modbus_cmds,
+	SHELL_CMD(set_baud, NULL, "Set Modbus baudrate", cmd_modbus_set_baud),
+	SHELL_CMD(set_id,   NULL, "Set Modbus slave ID", cmd_modbus_set_id),
+	SHELL_SUBCMD_SET_END
+);
+
+SHELL_STATIC_SUBCMD_SET_CREATE(sub_valve_cmds,
+    SHELL_CMD(set_open_time, NULL, "Set max valve opening time", cmd_valve_set_open_time),
+    SHELL_CMD(set_close_time, NULL, "Set max valve closing time", cmd_valve_set_close_time),
+    SHELL_SUBCMD_SET_END
+);
+
+SHELL_CMD_REGISTER(modbus, &sub_modbus_cmds, "Modbus configuration", NULL);
+SHELL_CMD_REGISTER(valve, &sub_valve_cmds, "Valve configuration", NULL);
+SHELL_CMD_REGISTER(show_config, NULL, "Show all configurations", cmd_config_show);