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>

en/blog/learning-slovene-where-to-start/index.html

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+    <link rel="stylesheet" href="../../../css/main.css?v=54203d0c">
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@@ -384,9 +384,9 @@
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 <!-- END_FOOTER -->
 
-    <script src="../../../main.js?v=cd97d474"></script>
+    <script src="../../../main.js?v=54203d0c"></script>
 
-<link rel="stylesheet" href="/assistant.css?v=cd97d474">
-<script src="/assistant.js?v=cd97d474" defer></script>
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 </body>
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