Upgrade to Hugo 0.16x and zen theme v7.1.0

- 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.

layouts/_partials/mobilemenu.html

@@ -8,7 +8,7 @@
       <rect width="80" height="12" x="10" y="70" rx="5"></rect>
     </svg>
   </button>
-  <div class="mobile-nav__sheet link-inverted link-nav" id="sheet" aria-hidden="true">
+  <div class="mobile-nav__sheet link-nav" id="sheet" data-theme="dark" aria-hidden="true">
     <div class="mobile-nav__region">{{ if hugo.IsMultilingual }}{{ partial "language-selector.html" . }}{{ end }}</div>
     <nav class="mobile-nav__main-menu" aria-label="{{ i18n "menu_title" }}">
     <ul class="mobile-nav__navbar">