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/partials/header.html

@@ -0,0 +1,11 @@
+<header class="site-header">
+  <a class="brand" href="{{ "/" | relLangURL }}">
+    <img src="/logo.png" alt="Tane" width="36" height="36" />
+    <span>Tane</span>
+  </a>
+  <nav class="site-nav">
+    {{ with .Site.GetPage "/about" }}<a href="{{ .RelPermalink }}">{{ T "navAbout" }}</a>{{ end }}
+    <a href="{{ "/legal/" | relLangURL }}">{{ T "navLegal" }}</a>
+    {{ partial "lang-switch.html" . }}
+  </nav>
+</header>