feat(templates): add Chat x402 template with Echo Credits / USDC payment switcher (closes #610)

[D3CC]

Description

Add Chat x402 template as requested in #610.

Reuses the existing AI elements from next-chat template and merges the x402 payment flow as specified by @rsproule.

What's included:

Auth/Payment Switcher:

• AuthSwitcher component allowing users to choose between Echo Credits and USDC (x402)
• Clean toggle UI consistent with Echo's design system
• Payment method passed to API route via request body

Chat Component (chat-x402.tsx):

• Reuses AI elements from next-chat template (Message, PromptInput, Loader)
• Integrated x402 payment error handling (HTTP 402)
• Payment-required UI feedback when funds are insufficient
• Dark mode support with Tailwind dark: classes

API Route (chat-x402/route.ts):

• x402 payment verification via @merit-systems/echo-aix402
• HTTP 402 response with payment headers when USDC payment is required
• Echo-billed AI providers for both payment methods
• Standard x402 headers: x402-payment-required, x402-accept, x402-amount, x402-network

Implementation Notes

• Built following rsproule's guidance: "reuse the existing one as much as possible. And merge the login flow from the sora template"
• Reuses Message, MessageContent, PromptInput, PromptInputTextarea, PromptInputSubmit, Loader from next-chat AI elements
• Echo SDK handles billing automatically for Echo Credits method
• x402 protocol handles USDC payment verification

Files

• templates/chat-x402/src/app/_components/chat-x402.tsx — Chat component with auth switcher
• templates/chat-x402/src/app/api/chat-x402/route.ts — API route with x402 payment handling

Closes #610

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