docs(api): align examples with client-side signing flow

[OperaCode]

Summary

• update api/README.md to document the current prepare/sign/submit API flow
• rewrite api/examples/usage.ts so it no longer sends userSecret to the server
• add an explicit note that Stellar secret keys must remain client-side

Context

Issue #88 references Keypair.fromSecret() handling in api/src/services/stellar.service.ts, but that code path is no longer present on main after PR #102 was merged on March 23, 2026.

Related to #88

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Issue	Title
#88	Fix: Add missing error handling for Keypair.fromSecret() in API	Link to this issue

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