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/home.html

@@ -0,0 +1,15 @@
+{{ define "title" }}{{ site.Title }}{{ end }}
+
+{{ define "main" -}}
+<main class="main layout__main">
+{{ .Content }}
+
+{{ $pages := .Site.RegularPages.RelatedTo (keyVals "tags" "current-event") -}}
+{{ $paginator := .Paginate $pages -}}
+{{ range $paginator.Pages -}}
+{{ .Render "summary" }}
+{{ end -}}
+
+{{ partial "pagination.html" . }}
+</main>
+{{ end -}}