Clean up Checkout use case page

- Remove AI-generated warning and marketing fluff
- Rewrite Overview with checkout-specific problem/solution framing
- Update to accurate metrics (9 EVM chains, 150+ currencies, 80+ wallets)
- Replace tabs with scannable cards for integration approaches
- Add 'When to Use Checkout' problem signal cards
- Replace speculative content with 4 concrete scenarios
- Remove non-existent features (Shopify plugins, code examples)
- Add Info callout for commerce escrow (in development but marketed)
- Update Key API Features to checkout-relevant features
- Correct demo URL to https://checkout.request.network
- Add tooltips for technical terms

Closes #33

Author: MantisClone