docs: unify buyer quick starts on wallet-cli 4.14.0

[boboliu-1010]

Summary

The buyer quick starts still required agent-wallet, private-key environment variables, and a TypeScript signing script. Replace both English and Chinese pages with the wallet-cli 4.14.0 buyer workflow: global installation, local account setup, Nile payment preview, authorized payment via password stdin, and structured result/error handling.

The example previews the seller guide's local GET /credit route with a 1 USDT cap and does not sign or pay by default. Preserve every previous rendered section anchor and the existing page URLs. SDK implementation details remain outside the default buyer setup.

Validation

• English and Chinese production builds passed with broken-link checks set to throw.
• All previous rendered IDs and both pages' local link destinations verified; no duplicate IDs.
• Seven discovery/help/schema commands verified against wallet-cli 4.14.0; shell example syntax checked.
• No agent-wallet or AGENT_WALLET references remain in these pages.
• No live payment or on-chain transaction was executed.