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.

assets/css/_custom.css

@@ -18,11 +18,15 @@ th, td {
   margin-left: 2em;
 }
 
+/* Zen v7 paints body with --color-body-bg and .page with --color-page-bg. */
+:root {
+    --color-body-bg: light-dark(var(--color-grey-100), #141414);
+    --color-page-bg: light-dark(var(--color-white), #242424);
+}
+
 @media (prefers-color-scheme: dark) {
     :root {
-        --color-backdrop: #141414;
         --color-headings: #faf5f5;
-        --color-body-bg: #242424;
         --color-header-bg: #242424;
         --color-text: #faf5f5;
         --color-link: #66f2ff;