feat: add campsites link to members area menu with membership access control

- Replace 'Coming Soon!' with 'Campsites' link in Members Area dropdown
- Add membership verification check to campsites.php
- Redirect non-logged-in users to login page
- Redirect non-members to index page
- Only active members can access campsites feature

header.php

@@ -299,7 +299,7 @@ if ($headerStyle === 'light') {
                                         <?php if ($is_member) : ?>
                                             <li class="dropdown"><a href="#">Members Area</a>
                                                 <ul>
-                                                    <li><a href="#">Coming Soon!</a></li>
+                                                    <li><a href="campsites">Campsites</a></li>
                                                 </ul>
                                             </li>
                                         <?php endif; ?>