Plan picker: prices in the visitor's currency, no VAT outside the EU

[helpcodeai]

Matches the marketing site's pricing page (website #397 / #398).

Stripe Checkout already charges visitors outside the euro area in their own currency (Adaptive Pricing), and only EU buyers pay VAT (our only Stripe Tax registration is DE). The plan picker on /start-trial and on the licence wall showed euros and "VAT included" to everyone.

What changes (Cloud only)

• lib/local-currency.ts:
  • useLocalPricing asks ${MARKETING_URL}/api/price-estimate once per page load, from the visitor's browser, so Cloudflare sees the visitor's country. The site allows this origin via CORS.
  • The site's answer is shape-checked; anything else counts as no answer.
• Plan picker:
  • Prices read ≈ $22.15/month with 19 €/month exactly under them.
  • Footer: "Prices in EUR, paid in USD at checkout." ("VAT included" only for EU visitors).
• /start-trial: the "billed today" lines show ≈ $22.15/month (19 €/month).
• Unchanged:
  • euro-area visitors;
  • unknown countries;
  • self-hosted (no request is made);
  • an unreachable site: euros and "VAT included" as before.

Tests

• Playwright card-trial.spec.ts: 25 passed, including 5 new:
  • helpers;
  • a US visitor (dollars first, exact euros under them, no VAT, yearly);
  • a euro visitor;
  • the site unreachable.
• billing-awareness and neutral-signup-and-billing still pass.
• tsc clean, eslint 0 errors.

Depends on website #398 (CORS for the Cloud origin and the vat field) being live first.