feat: add a German page at /de/

[PhilippTheServer]

Closes #5

• de/index.md: full German version (lang: de), targeting Open-Source-Shopsystem, selbst gehostet, Webshop, Shopware-Alternative, Headless-E-Commerce.
• head.html: hreflang alternates for en, de and x-default on every page; og:locale / og:locale:alternate per language.
• default.html: <html lang> from the page; a Deutsch/English language switch in the header; the few UI strings (skip link, brand note, footer) translated.
• README documents the two-language setup; llms.txt points to /de/.
• check_site.rb fails if the German page, sitemap entry, lang/og:locale, hreflang alternates, language switch or German search terms are lost.

Verification

./scripts/verify.sh (Ruby 3.3 container)

before the page existed:
check_site.rb: 6 failure(s)
  ✗ de/index.html: missing from the built site
  ✗ sitemap.xml: does not contain the German page
  ✗ index.html: has lost the hreflang=en alternate
  ✗ index.html: has lost the hreflang=de alternate
  ✗ index.html: has lost the hreflang=x-default alternate
  ✗ index.html: has lost the language switch to /de/

after:
HTML-Proofer finished successfully.
check_site.rb: all checks passed
==> all verification passed

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