fix: replace deprecated languageCode/Site.LanguageCode with locale

Hugo v0.158.0 deprecated both in favor of the locale config key and
.Site.Language.Locale, matching pivoine.art's config.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3xLyuoN6uzfCYBo8kVoSn

layouts/_default/baseof.html

@@ -1,5 +1,5 @@
 <!doctype html>
-<html lang="{{ .Site.LanguageCode }}" class="scroll-smooth">
+<html lang="{{ .Site.Language.Locale }}" class="scroll-smooth">
   <head>
     {{- partial "head.html" . -}}
   </head>