feat: Stripe Checkout payment plugin 1.0.0

- Stripe Checkout (hosted page) through the shop processOrder() seam; REST via
  Laravel Http, no stripe/stripe-php dependency; pinned Stripe-Version.
- One consolidated line for the order total; zero/three-decimal currency rules.
- One idempotent fulfilment path shared by the return page and the signed
  webhook (payment intent / refund id as ledger keys, unique-index race handled);
  order status moved through changeStatus() only while the order is still new.
- Refunds made in Stripe are recorded on the order; only a full refund moves it
  to the refunded status.
- Webhook route outside the storefront middleware (maintenance page / redirects
  would swallow it) and rate limited; HMAC v1 signature, 300 s tolerance.
- Cancel link never cancels a paid or already-handled order.
- Per-store settings with encrypted secrets, owner-only; settings screen split
  into Mode / Sandbox / Live / Order status blocks.
- README (vi/en) with a step-by-step guide to creating Stripe keys and webhooks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>