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/baseof.html

@@ -1,6 +1,6 @@
 <!doctype html>
 <html
-  lang="{{ with site.Params.isoCode | default (site.LanguageCode | default `en`) }}{{- . -}}{{ end }}"
+  lang="{{ with site.Params.isoCode | default (site.Language.Locale | default `en`) }}{{- . -}}{{ end }}"
   dir="{{ cond (site.Params.rtl | default false) `rtl` `ltr` }}"
   class="scroll-smooth"
   data-default-appearance="{{ site.Params.defaultAppearance | default `light` }}"