feat: turn opentaberna.de into a customer landing page

[PhilippTheServer]

Closes #10

What changes

• Home (/, /de/): hero with real storefront and back-office screenshots; trust strip; what a shop gets (two screenshot rows and six capabilities); three ways to run it (do it yourself, set up on your server, hosted in Germany); custom storefront; Shopware/Shopify comparison citing the vendors' pricing pages; FAQ; closing call to action; founders. Both languages render one template (_includes/landing.html) from _data/i18n.yml.
• Calls to action: pre-filled mailto:root@vaultops.de links with a subject and a short questionnaire per offer (general, setup, hosting, custom storefront). No form, no third party.
• Developers (/developers/, /de/entwickler/): repositories, stack, docs and licence moved here from the old home page. The licence text no longer says the founders don't want to earn money.
• Legal: /impressum/ (§ 5 DDG, § 18 Abs. 2 MStV, vaultops details) and /datenschutz/ (GitHub Pages hosting, EU-US Data Privacy Framework, email contact, rights); linked from every footer.
• Screenshots: assets/img/*.webp (185 KB total), real apps on the dev stack with the sample catalogue; og.jpg social preview.
• Design: new stylesheet with the same brand colours; sticky header with a CTA; works at 390 px; dark mode. Still no JavaScript and nothing loaded from other origins.
• llms.txt and README updated.

Verification

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

with the new checks, before the redesign (first commit on this branch):
check_site.rb: 45 failure(s)
  ✗ developers/index.html: missing from the built site
  ✗ impressum/index.html: missing from the built site
  ✗ index.html: footer has lost the link to /impressum/
  ✗ index.html: has lost the #offers section
  ✗ index.html: needs pre-filled emails for setup, hosting, custom frontend and a general enquiry, found 0
  ✗ index.html: comparison has lost its source https://www.shopware.com
  …

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

Browser checks (Playwright, local build): no horizontal scroll at 390 px on any of the six pages; lazy screenshots load; light and dark mode reviewed on desktop and mobile.

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