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/shortcodes/lang.html

@@ -1,16 +0,0 @@
-{{- $code := .Get 0 | lower -}}
-{{- $validCodes := slice "de" "en" "deen" -}}
-{{- if not (in $validCodes $code) -}}
-  <span style="color: red; font-weight: bold;">Warning: Invalid or missing language code '{{ $code }}'. Use 'de', 'en', or 'deen'.</span>
-{{- else -}}
-  {{- $label := cond (eq $code "de") "DE" (cond (eq $code "en") "EN" (cond (eq $code "deen") "DE/EN" "")) -}}
-  {{- $title := "" -}}
-  {{- if eq $code "deen" -}}
-      {{- $title = cond (eq .Site.Language.Lang "de") "Diese Tour wird zweisprachig stattfinden (Deutsch und Englisch)" "This tour will be bilingual (German and English)" -}}
-  {{- else if eq $code "de" -}}
-      {{- $title = cond (eq .Site.Language.Lang "de") "Diese Tour wird auf Deutsch stattfinden" "This tour will be held in German" -}}
-  {{- else if eq $code "en" -}}
-      {{- $title = cond (eq .Site.Language.Lang "de") "Diese Tour wird auf Englisch stattfinden" "This tour will be held in English" -}}
-  {{- end -}}
-  <abbr style="font-size: .65em; vertical-align: super;" title="{{ $title }}">{{ $label }}</abbr>
-{{- end -}}