Refactor navigation to Tabs+Languages, add contact page and Inter font

Switch primary navigation to Tabs containing Languages for multi-topic
extensibility. Add contact page with project links, issue reporting,
and contribution guide. Set Inter as the sans-serif font for English
text and add custom CSS for description sizing and logo rendering.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

style.css

@@ -6,3 +6,8 @@ img[src*="favicon"] {
   height: auto !important;
   width: auto !important;
 }
+
+/* Page description (frontmatter) - match body text size */
+header .text-lg {
+  font-size: 1rem;
+}