Add /die-sieben — rebuilt from the old WordPress page, brand-voice copy

Free monthly ritual page, modeled on /todo-cards' conventions
(Reveal/RevealGroup, breadcrumb + text-display heading) but without a
product-style hero — this isn't a paid product, so no photo/price/buy
CTA. The WordPress original's monthly-changing "aktuelle Ausgabe",
printable template, and archive have no content model in this codebase
yet (nothing like a `Posts` collection for "editions") — this ships the
evergreen concept page only, with the existing Newsletter signup
standing in as the "stay in the loop" mechanism. Deferred to a memory
note rather than blocking this page on designing a new collection.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018unaXmuzVA8ct1b6WoyP1U

README.md

@@ -88,6 +88,7 @@ not in a committed `.env`.
 | `/konto/merkliste` | Wishlist (when `wishlistEnabled`, see "Feature toggles" below) |
 | `/challenge` | "Mini-Challenge" tool |
 | `/todo-cards` | "Todo-Karten" tool |
+| `/die-sieben` | "Die Sieben" — free monthly ritual (evergreen concept page only; the WordPress original's monthly-changing edition/archive/template has no content model in this codebase yet, see `app/die-sieben/page.tsx`'s own comment) |
 | `/newsletter` | Newsletter signup |
 | `/versand` | Shipping policy (timeframes, costs) |
 | `/impressum`, `/datenschutz`, `/agb`, `/widerruf` | Legal pages (from Payload, see `legal-pages` below) |