Nav cleanup + new Blog page

Remove 'Mentorship Programme' and 'Open calls' from the menus, rename 'Announcements' to 'News and Announcements', and wire the Blog link. Add a trilingual /<lang>/blog/ page with a distinct look: featured strip, client-side topic filters, and a masonry card wall with colour-coded tags (placeholder posts for now).

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

css/main.css

@@ -33,4 +33,5 @@
 @import 'pages/_about.css';
 @import 'pages/_timeline.css';
 @import 'pages/_guide.css';
-@import 'pages/_faq.css';
+@import 'pages/_faq.css';
+@import 'pages/_blog.css';