feat(web): tane.comunes.org landing + localized app screenshots

- Hugo site (site/): EN/ES intro + legal (generated from docs/legal), an
  ES/EN About page from docs/{que-es-tane,what-is-tane}.md, seed-green theme,
  responsive header, store-badge placeholders, multi-stage Hugo->nginx image.
- Golden screenshot harness (test/screenshots/): en,es,fr,de,pt,ja + RTL demo,
  real fonts via DejaVu Sans + FontManifest; skip-by-default tag so CI stays
  green. collect_screenshots.sh feeds site/ and fastlane phoneScreenshots.
- Drop editorial notes from the public explainer.

site/layouts/404.html

@@ -0,0 +1,7 @@
+{{ define "main" }}
+<section class="doc">
+  <h1>404</h1>
+  <p class="lead">This page could not be found.</p>
+  <p class="crumb"><a href="{{ "/" | relLangURL }}">← {{ T "backToHome" }}</a></p>
+</section>
+{{ end }}