Add /newsletter-confirmed double-opt-in confirmation page

Brevo's redirectionUrl now points here instead of the homepage — a
static page matching /bestellbestaetigung's visual language (brand-
tinted checkmark circle, serif display heading, thin brand divider).
No query params to read; Brevo's confirmation redirect carries nothing
this page needs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -56,7 +56,7 @@ independent, see the "Monitoring & alerting" section). `STRIPE_SECRET_KEY`,
 (Stripe)" below. `BREVO_API_KEY`, `BREVO_LIST_ID`,
 `BREVO_DOUBLE_OPTIN_TEMPLATE_ID` (no safe default — required for
 newsletter signups to trigger a confirmation email at all),
-`BREVO_DOI_REDIRECT_URL` (optional, defaults to the homepage) — see
+`BREVO_DOI_REDIRECT_URL` (optional, defaults to `/newsletter-confirmed`) — see
 "Newsletter signup" below. Set in Coolify's app settings for production,
 not in a committed `.env`.
 
@@ -1074,9 +1074,13 @@ unsynced.
   (`POST /v3/contacts/attributes/normal/OPT_IN_SOURCE`) or Brevo silently
   drops it on every request (no error at all, just never stored) rather
   than rejecting the request. `redirectionUrl` (where Brevo sends the
-  contact after they click confirm) defaults to the homepage via
-  `BREVO_DOI_REDIRECT_URL` — no dedicated "danke, bestätigt" landing page
-  exists yet.
+  contact after they click confirm) defaults to `/newsletter-confirmed`
+  via `BREVO_DOI_REDIRECT_URL` — a static confirmation page
+  (`app/newsletter-confirmed/page.tsx`), same visual language as
+  `/bestellbestaetigung` (brand-tinted circular checkmark, serif display
+  heading, thin brand divider). No query params to read — Brevo's
+  redirect carries nothing this page needs, unlike `/checkout/verarbeitung`
+  which polls actual payment status.
 - **`app/lib/useNewsletterSignup.ts`** — the shared email/consent/submit
   state + on-blur validation + refocus-on-invalid-submit behind all four
   forms (same "state of the art, simple" input-quality bar as checkout's