feat(ui): replace alert notifications with toast messages

Add toast notification functionality using Bootstrap Toasts and update
checkout success/error handling to use toast messages instead of alert boxes.

**Changes:**
- Add `showToast` function to `terminal.js`
- Add toast container and templates to `terminal.html`
- Replace alert() calls in handleCheckoutSubmit with showToast()

myproject/pos/templates/pos/terminal.html

@@ -729,6 +729,28 @@
 
 <!-- Модалка редактирования товара в корзине -->
 {% include 'pos/components/edit_cart_item_modal.html' %}
+
+<!-- Toast Container для уведомлений -->
+<div class="toast-container position-fixed top-0 end-0 p-3" style="z-index: 1060;">
+  <div id="orderSuccessToast" class="toast align-items-center border-0" role="alert" aria-live="assertive" aria-atomic="true">
+    <div class="d-flex">
+      <div class="toast-body">
+        <i class="bi bi-check-circle-fill text-success me-2 fs-5"></i>
+        <span id="toastMessage"></span>
+      </div>
+      <button type="button" class="btn-close btn-close-white me-2 m-auto" data-bs-dismiss="toast" aria-label="Close" style="display: none;"></button>
+    </div>
+  </div>
+
+  <div id="orderErrorToast" class="toast align-items-center border-0" role="alert" aria-live="assertive" aria-atomic="true">
+    <div class="d-flex">
+      <div class="toast-body">
+        <i class="bi bi-exclamation-circle-fill text-danger me-2 fs-5"></i>
+        <span id="errorMessage"></span>
+      </div>
+    </div>
+  </div>
+</div>
 {% endblock %}
 
 {% block extra_js %}