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/summary.html

@@ -0,0 +1,12 @@
+<article class="list-view{{ with .Section }} list-view--{{ . | urlize }}{{ end }}">
+<header>
+{{- $postDate := "" }}
+{{- if .Param "showPostDate" | default false }}
+{{- $postDate = .Params.date.Format .Site.Params.dateFormat }}
+{{- end }}
+<h2 class="title mt--s{{ if $.Param "submitted" | default false }} mb--xxs{{ end }}"><a href="{{ .RelPermalink }}">{{ .Title }}</a> <span class="title-post-date">{{ $postDate }}</span></h2>
+{{ if $.Param "submitted" | default false }}{{ partial "submitted.html" . -}}{{ end }}
+</header>
+
+{{ .Summary }}
+</article>