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

@@ -0,0 +1,61 @@
+.header__intro_headline {
+  font: bold var(--fs-xxl) "Arial";
+  padding-bottom: 10px;
+}
+.header__intro_text {
+  font-family: var(--ff-body)
+}
+table, th, td {
+    border: 1px solid;
+  }
+th, td {
+    padding: 3px 6px;
+    font-variant-numeric: tabular-nums;
+}
+
+.title-post-date {
+  font-size: .7em;
+  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-headings: #faf5f5;
+        --color-header-bg: #242424;
+        --color-text: #faf5f5;
+        --color-link: #66f2ff;
+        --color-footer-bg: #242424;
+        --color-border: #faf5f5;
+        --color-button: #141414;
+        --color-button-text: #faf5f5;
+        --color-button-hover: #343434;
+        --color-brand: #66f2ff;
+        --color-text-meta: #faf5f5;
+        --color-mobile-menu: #141414;
+    }
+
+    nav ul li {
+        flex-grow: 1;
+        text-align: center;
+    }
+    .main-menu a:active {
+        text-decoration: none;
+    }
+    a {
+        text-decoration: none;
+    }
+}
+
+h2 a {
+  margin-right: 2em;
+}
+
+.title-post-date {
+  margin-left: 0em;
+}