[shadowmap] feat: add marketing landing page with Stripe checkout

[mdabir1203]

Summary

• add a Stripe-ready marketing landing page with USA/EU pricing, feature highlights, and CTA copy tuned for conversions
• wire /create-checkout-session to Stripe Checkout using region-aware price IDs and expose the recon dashboard at /app
• document the required Stripe environment variables for running the billing flow locally

Testing

• cargo fmt --all
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

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https://chatgpt.com/codex/tasks/task_e_68eb5afe7548832696211b1f76ed31fe