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/list-bytitle.html

@@ -0,0 +1,16 @@
+{{ define "main" -}}
+<main class="main layout__main">
+<header>
+<h1 class="title">{{ .Title }}</h1>
+</header>
+
+{{ .Content }}
+
+{{ $paginator := .Paginate (.Pages) -}}
+{{ range $paginator.Pages.ByTitle -}}
+{{ .Render "summary" }}
+{{ end -}}
+
+{{ partial "pagination.html" . }}
+</main>
+{{ end -}}