fix(a11y): activities tablist must contain only tabs

The "Become a member" CTA was a direct child of role="tablist", which
violates the ARIA rule that a tablist may only contain role="tab" children
(axe: aria-required-children). Wrap the 4 tabs in an inner
.activities-tabs[role=tablist] and leave the CTA as a sibling; add matching
flex CSS so the layout is unchanged. JS uses `.activities-nav .activity-tab`
(descendant), so tab behavior is unaffected. EN/MK/SI. Cache-bust bump.

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

404.html

@@ -25,7 +25,7 @@
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
     <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap" rel="stylesheet">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css" crossorigin="anonymous" referrerpolicy="no-referrer">
-    <link rel="stylesheet" href="/css/main.css?v=cd97d474">
+    <link rel="stylesheet" href="/css/main.css?v=54203d0c">
 
     <link rel="icon" type="image/png" href="/favicon-96x96.png" sizes="96x96">
     <link rel="icon" type="image/svg+xml" href="/favicon.svg">
@@ -70,7 +70,7 @@
             </p>
         </div>
     </main>
-<link rel="stylesheet" href="/assistant.css?v=cd97d474">
-<script src="/assistant.js?v=cd97d474" defer></script>
+<link rel="stylesheet" href="/assistant.css?v=54203d0c">
+<script src="/assistant.js?v=54203d0c" defer></script>
 </body>
 </html>