Add hosted checkout page for direct payment collection

[Smartdevs17]

Description

Build a hosted checkout page that merchants can link customers to for payment collection, supporting all payment methods with a responsive, optimized UI.

Acceptance Criteria

• Hosted checkout page with merchant branding
• Payment method selection (crypto, fiat, wallet)
• Real-time exchange rate display with lock option
• Mobile-optimized responsive design
• Post-payment confirmation with receipt download
• Abandoned checkout recovery via email reminder

Technical Scope

• frontend/app/forms/embed/[id]/page.tsx
• frontend/components/forms/FormRenderer.tsx
• backend/src/services/forms.ts
• Edge: browser back button handling, payment timeout, concurrent session checkout