Update sitemaps and improve language handling

- Updated last modification dates in static and tools sitemaps to 2026-04-01.
- Enhanced language switching in the Header component to ensure language resources are loaded before changing the language.
- Added language resource loading logic in i18n configuration to support dynamic loading of language files.
- Improved SEO route page to ensure correct language is set based on URL parameters.
- Adjusted global CSS for deferred sections to optimize rendering.
- Configured Nginx to enable Brotli compression for better performance.

nginx/nginx.prod.conf

@@ -65,6 +65,11 @@ server {
     gzip_types text/plain text/css application/json application/javascript text/xml application/xml text/javascript image/svg+xml;
     gzip_min_length 1000;
 
+    # Brotli (if module is available)
+    brotli on;
+    brotli_comp_level 5;
+    brotli_types text/plain text/css application/json application/javascript text/xml application/xml image/svg+xml;
+
     # SSE streaming for assistant chat
     location /api/assistant/chat/stream {
         proxy_pass http://$backend_upstream;