fix: migrate deprecated Hugo v0.158.0 language API

Replace deprecated config keys and template methods per Hugo v0.158.0:

Config (10 files):
- languageCode → locale
- languageName → label

Templates:
- site.LanguageCode → site.Language.Locale (baseof.html, head.html, schema.html x2)
- site.Language.Lang → site.Language.Name (single.html)
- .Lang → .Language.Name (likes.html, views.html)

Ref: https://discourse.gohugo.io/t/deprecations-in-v0-158-0/56869

layouts/_default/single.html

@@ -123,7 +123,7 @@
       {{ end }}
 
       {{ $finalLink := $taxonomyLink }}
-      {{ $currentLang := site.Language.Lang }}
+      {{ $currentLang := site.Language.Name }}
       {{ if eq site.LanguagePrefix "" }}
         {{ $finalLink = printf "%sauthors/%s/" $baseURL $author }}
       {{ else }}