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>

si/index.html

@@ -71,7 +71,7 @@
                 </ul>
             </nav>
             <div class="navigation-actions">
-                <button class="btn btn-secondary">Postani član</button>
+                <a href="../si/become-a-member/" class="btn btn-secondary">Postani član</a>
                 <button class="btn btn-primary">Doniraj</button>
                 <div class="language-selector">
                     <div class="current-lang">
@@ -141,7 +141,7 @@
         </ul>
     </nav>
     <div class="navigation-actions">
-        <button class="btn btn-secondary">Postani član</button>
+        <a href="../si/become-a-member/" class="btn btn-secondary">Postani član</a>
         <button class="btn btn-primary">Doniraj</button>
         <div class="language-selector">
             <button class="current-lang" id="mobile-lang-trigger">
@@ -186,7 +186,7 @@
                     <h1>Opolnomočimo mlade z veščinami za spreminjanje sveta</h1>
                     <p>Ne glede na to, ali imate ekipo 2 ali 200 ljudi, naši skupni timski nabiralniki ohranjajo vse na isti strani in v koraku z dogajanjem.</p>
                 </div>
-                <button class="btn btn-primary-orange">Postani član</button>
+                <a href="../si/become-a-member/" class="btn btn-primary-orange">Postani član</a>
             </div>
         </section>
 
@@ -252,7 +252,7 @@
                         <li>Zabava in druženje</li>
                         <li>Študentska podpora</li>
                     </ul>
-                    <button class="btn btn-primary-orange">Postani član</button>
+                    <a href="../si/become-a-member/" class="btn btn-primary-orange">Postani član</a>
                 </div>
                 <div class="activity-details">
                     <p>Sed molestie penatibus sit bibendum pharetra purus faucibus fames aliquet. Morbi morbi ac sed tempor porta. Dui iaculis vitae fames sapien. Pellentesque urna eget habitasse arcu sed ornare. Dignissim lobortis ultrices maecenas magna ac habitant donec etiam at.</p>