feat: implement order management and checkout functionality

- Added `Order` and `OrderItem` models with migrations for managing orders and their details
- Created `CheckoutForm` for validating order data and customer information
- Built templates for order confirmation, checkout success, and emails (HTML and plain text)
- Added logic for email notifications upon successful order placement
- Enhanced shopping cart template with checkout panel integration
- Updated styles and JavaScript for streamlined checkout UX
- Refactored cart view to support item summaries and final totals during checkout

templates/orders/emails/order_confirmation_email.html

@@ -0,0 +1,38 @@
+<div style="max-width:640px;margin:0 auto;background:#ffffff;border:1px solid #d6dde6;padding:32px;font-family:Arial,sans-serif;color:#36213e;">
+    <h1 style="margin:0 0 16px;font-size:28px;">Заказ №{{ order.id }} оплачен</h1>
+    <p>Здравствуйте, {{ order.customer_name }}!</p>
+    <p>Ваш заказ успешно оформлен и оплачен.</p>
+
+    <table style="width:100%;border-collapse:collapse;margin:24px 0;">
+        <thead>
+        <tr>
+            <th style="text-align:left;border-bottom:1px solid #d6dde6;padding:8px;">Товар</th>
+            <th style="text-align:right;border-bottom:1px solid #d6dde6;padding:8px;">Кол-во</th>
+            <th style="text-align:right;border-bottom:1px solid #d6dde6;padding:8px;">Сумма</th>
+        </tr>
+        </thead>
+        <tbody>
+        {% for item in order.items.all %}
+            <tr>
+                <td style="padding:8px;border-bottom:1px solid #eef2f6;">{{ item.item_name }}</td>
+                <td style="padding:8px;border-bottom:1px solid #eef2f6;text-align:right;">{{ item.quantity }}</td>
+                <td style="padding:8px;border-bottom:1px solid #eef2f6;text-align:right;">{{ item.line_total }} руб.</td>
+            </tr>
+        {% endfor %}
+        </tbody>
+    </table>
+
+    <p><strong>Товары:</strong> {{ order.products_total }} руб.</p>
+    <p><strong>Скидка:</strong> {{ order.discount_total }} руб.</p>
+    <p><strong>Доставка:</strong> {{ order.delivery_total }} руб.</p>
+    <p><strong>Подъём на этаж:</strong> {{ order.floor_lift_total }} руб.</p>
+    <p style="font-size:18px;"><strong>Итого к оплате:</strong> {{ order.total }} руб.</p>
+
+    <p><strong>Доставка:</strong> {{ order.get_delivery_method_display }}</p>
+    {% if order.region or order.address %}
+        <p><strong>Адрес:</strong> {{ order.region }}{% if order.address %}, {{ order.address }}{% endif %}</p>
+    {% endif %}
+    <p><strong>Оплата:</strong> {{ order.get_payment_method_display }}</p>
+
+    <p style="margin-top:24px;color:#554971;">Спасибо за покупку в {{ site_name }}.</p>
+</div>