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/valve/CMakeLists.txt

@@ -0,0 +1 @@
+zephyr_library_sources(valve.c)

software/lib/valve/Kconfig

@@ -0,0 +1,5 @@
+config LIB_VALVE
+    bool "Enable Valve Library"
+    default y
+    help
+      Enable the Valve Library.

software/lib/valve/valve.c

@@ -0,0 +1,62 @@
+#include <zephyr/kernel.h>
+#include <zephyr/settings/settings.h>
+#include <zephyr/logging/log.h>
+#include <lib/valve.h>
+
+LOG_MODULE_REGISTER(valve, LOG_LEVEL_INF);
+
+static enum valve_state current_state = VALVE_STATE_CLOSED;
+static enum valve_movement current_movement = VALVE_MOVEMENT_IDLE;
+static uint16_t max_opening_time_s = 60;
+static uint16_t max_closing_time_s = 60;
+static struct k_work_delayable valve_work;
+
+static void valve_work_handler(struct k_work *work)
+{
+    if (current_movement == VALVE_MOVEMENT_OPENING) {
+        LOG_INF("Virtual valve finished opening");
+    } else if (current_movement == VALVE_MOVEMENT_CLOSING) {
+        current_state = VALVE_STATE_CLOSED;
+        LOG_INF("Virtual valve finished closing");
+    }
+    current_movement = VALVE_MOVEMENT_IDLE;
+}
+
+void valve_init(void)
+{
+    k_work_init_delayable(&valve_work, valve_work_handler);
+    settings_load_one("valve/max_open_time", &max_opening_time_s, sizeof(max_opening_time_s));
+    settings_load_one("valve/max_close_time", &max_closing_time_s, sizeof(max_closing_time_s));
+}
+
+void valve_open(void)
+{
+    if (current_state == VALVE_STATE_CLOSED) {
+        current_state = VALVE_STATE_OPEN;
+        current_movement = VALVE_MOVEMENT_OPENING;
+        k_work_schedule(&valve_work, K_SECONDS(max_opening_time_s));
+    }
+}
+
+void valve_close(void)
+{
+    if (current_state == VALVE_STATE_OPEN) {
+        current_movement = VALVE_MOVEMENT_CLOSING;
+        k_work_schedule(&valve_work, K_SECONDS(max_closing_time_s));
+    }
+}
+
+void valve_stop(void)
+{
+    k_work_cancel_delayable(&valve_work);
+    current_movement = VALVE_MOVEMENT_IDLE;
+}
+
+enum valve_state valve_get_state(void) { return current_state; }
+enum valve_movement valve_get_movement(void) { return current_movement; }
+uint16_t valve_get_motor_current(void) { return (current_movement != VALVE_MOVEMENT_IDLE) ? 150 : 10; }
+
+void valve_set_max_open_time(uint16_t seconds) { max_opening_time_s = seconds; settings_save_one("valve/max_open_time", &max_opening_time_s, sizeof(max_opening_time_s)); }
+void valve_set_max_close_time(uint16_t seconds) { max_closing_time_s = seconds; settings_save_one("valve/max_close_time", &max_closing_time_s, sizeof(max_closing_time_s)); }
+uint16_t valve_get_max_open_time(void) { return max_opening_time_s; }
+uint16_t valve_get_max_close_time(void) { return max_closing_time_s; }