Align Stripe checkout redirects with v8

[dayhaysoos]

Summary

• update checkout redirects to use Stripe session URLs while keeping a legacy sessionId fallback
• align React/Stripe/testing dependencies with current releases and Stripe v8 requirements
• refresh docs and type examples for the updated checkout flow

Testing

• pnpm --filter use-shopping-cart run test
• pnpm --filter use-shopping-cart run test:types

[cursor]

PR Summary

Aligns checkout with Stripe v8 using session.url

• Core: Add flexible redirectToCheckout(input) that accepts a session.url or legacy sessionId; implement browser redirect for URLs and Stripe.js fallback for IDs; validate publishable key format; expose types in core and react d.ts
• Tests: New/updated unit tests in core/stripe.test.ts and ShoppingCart.test.ts for missing input, key validation, and browser-only behavior
• Docs: Replace sessionId flow with sessionUrl across guides (actions, hooks, getting-started, serverless examples) and Stripe references; clarify notes about Stripe.js v8
• Examples: Update client/server snippets to return/use { sessionUrl } and call redirectToCheckout(sessionUrl)
• Site: Minor UI tweak in HeroShowcase padding and remove old badge from home; update badges; wording in payment-providers.md
• Deps: Bump docs React/React DOM and type packages to 19.2.3

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