web: official ensō+key icon, Kaizenkodo link, installable home-screen app

- App icon = the official Kaizenkodo ensō (logo/current/exports/tight) with a key
  inside: cream bg for the home-screen/app icon, transparent for the header +
  favicons. Source: assets/enso-base.png + icon-source.svg / enso-mark.svg.
- Attribution: a 'by Kaizenkodo' header link and a footer line, both to kaizenkodo.no.
- PWA install: manifest (standalone, start_url/scope '../', maskable icons) + iOS
  metas (apple-mobile-web-app-capable/title/status-bar) so it adds to the iPhone
  home screen and runs as a standalone app.

helwasm/assets/enso-mark.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512">
+<image href="enso-base.png" x="25.6" y="18.8" width="460.8" height="474.4"/>
+<g><circle cx="256" cy="189" r="30" fill="none" stroke="#1b4161" stroke-width="22"/><rect x="246" y="211" width="20" height="120" rx="4" fill="#1b4161"/><rect x="266" y="288" width="28" height="13" rx="3" fill="#1b4161"/><rect x="266" y="310" width="18" height="13" rx="3" fill="#1b4161"/></g>
+</svg>