Build landing page from Penpot design, with multi-language switcher

Implement the "Desktop dark" Penpot design: nav, hero, about, products
teaser and contact form, with the exported logo and wave artwork and
self-hosted Poppins, Roboto Flex and Chillax fonts.

- Shared page markup in _includes/home.pug; copy in locales/{en,pl}.json
- Footer language switcher and hreflang tags generated from the locales
- Contact form composes a mailto: to the address in _data/site.json
- Stacked layout below 1024px

README.md

@@ -3,10 +3,19 @@
 Landing page for continuum-3d.com. Eleventy + Pug + Tailwind CSS, with a
 small JSON-based i18n setup (`src/locales/en.json`, `src/locales/pl.json`).
 
-- English lives at `src/en/index.pug` and builds to `/`.
-- Polish lives at `src/pl/index.pug` and builds to `/pl/`.
-- Add more languages by adding `src/locales/<lang>.json` and a
-  `src/<lang>/index.pug` with `lang: <lang>` in its front matter.
+- English (the default language) lives at `src/en/index.pug` and builds to `/`.
+- Every other language lives at `src/<lang>/index.pug` and builds to `/<lang>/`
+  (e.g. Polish: `src/pl/index.pug` -> `/pl/`).
+- The page markup is shared in `src/_includes/home.pug`; all copy is in
+  `src/locales/<lang>.json`.
+- To add a language: copy `src/locales/en.json` to `src/locales/<lang>.json`,
+  translate it (including `lang_name`, the language's own name), and add a
+  `src/<lang>/index.pug` with `lang: <lang>` and `permalink: /<lang>/` in its
+  front matter that includes `../_includes/home.pug`. The footer language
+  switcher and the `hreflang` tags pick it up automatically.
+- Site-wide settings (domain, contact address, default language) are in
+  `src/_data/site.json`. The contact form opens the visitor's mail client
+  addressed to `contactEmail`.
 
 ## Develop