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.

.forgejo/workflows/deploy.yaml

@@ -11,7 +11,7 @@ jobs:
   build:
     runs-on: docker
     container:
-      image: docker.io/hugomods/hugo:0.150.1
+      image: docker.io/hugomods/hugo:0.165.0
     steps:
       - name: Pipeline info
         run: |