Add flagship Become a Member page with interactive multi-step form

New trilingual /<lang>/become-a-member/ conversion page: hero with real-photo collage, why-join benefits, animated MSOS-in-numbers, ways to participate, membership-in-real-life and projects showcase (real project photos), testimonials, who-can-join, an adaptive multi-step form (student + supporter flows, conditional fields, progress, personalised messages, validation, success screen), FAQ and final CTA. Site-wide 'Become a member' buttons now link here. Form posts to a Google Form once wired; metrics/testimonials are placeholders to confirm.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

en/index.html

@@ -71,7 +71,7 @@
                 </ul>
             </nav>
             <div class="navigation-actions">
-                <button class="btn btn-secondary">Become a member</button>
+                <a href="../en/become-a-member/" class="btn btn-secondary">Become a member</a>
                 <button class="btn btn-primary">Donate</button>
                 <div class="language-selector">
                     <div class="current-lang">
@@ -141,7 +141,7 @@
         </ul>
     </nav>
     <div class="navigation-actions">
-        <button class="btn btn-secondary">Become a member</button>
+        <a href="../en/become-a-member/" class="btn btn-secondary">Become a member</a>
         <button class="btn btn-primary">Donate</button>
         <div class="language-selector">
             <button class="current-lang" id="mobile-lang-trigger">
@@ -183,7 +183,7 @@
                     <h1>Empowering young people with skills to change the world</h1>
                     <p>Be part of our exciting journey; Become a part of our student organization today!</p>
                 </div>
-                <button class="btn btn-primary-orange">Become a member</button>
+                <a href="../en/become-a-member/" class="btn btn-primary-orange">Become a member</a>
             </div>
         </section>
 
@@ -249,7 +249,7 @@
                         <li>Education & Culture</li>
                         <li>Student Representation</li>
                     </ul>
-                    <button class="btn btn-primary-orange">Become a member</button>
+                    <a href="../en/become-a-member/" class="btn btn-primary-orange">Become a member</a>
                 </div>
                 <div class="activity-details">
                     <p>We know moving to a new country can feel overwhelming, so we’re here to make it easier. From helping you with university applications to finding your way around student life in Slovenia, you can always count on us. We offer free consultations, answer your questions on Instagram and Facebook (yes, even late at night), and host events where you can meet others and hear real experiences first-hand. With mentorship, info sessions, and a friendly community by your side, you’ll never have to go through the journey alone.</p>