- Bump themes/zen from v5.7.0 to v7.1.0 (vanilla CSS, no libsass).
- Move layouts to Hugo's new template structure (_default -> root,
index.html -> home.html, shortcodes -> _shortcodes).
- Replace deprecated language API in templates (Locale, Direction, Name)
and sync base templates and mobile menu with theme v7.
- Config: languageName/languageCode -> label/locale; move copyright to
the top-level key the v7 footer reads.
- Port _custom.scss to assets/css/_custom.css and map page/body
backgrounds to v7's --color-body-bg/--color-page-bg.
- CI: build with hugomods/hugo:0.165.0.
