refactor: update styles and structure for header, navigation, and banner

- Removed unnecessary "active-page" class from banner link
- Added reusable "nav-block" class for header links
- Adjusted `.nav-links` structure: switched to `inline-flex`, added `justify-content`
- Reduced `.kitchen-banner` margin and added border, padding for text styles
- Enhanced flex layout for `home-index-block` in index.css

templates/home/_fragments/banner.html

@@ -2,7 +2,7 @@
     <div class="kitchen-content">
         <p>Успей заказать с большой выгодой и получите доставку в подарок!</p>
         <h2>ГОТОВОЕ РЕШЕНИЕ ДЛЯ КУХНИ</h2>
-        <a href="#" class="active-page">Перейти</a>
+        <a href="#">Перейти</a>
     </div>
     {#    <img src="{% static 'img/kitchen-banner.jpg' %}" alt="">#}
 </section>