7. Add `lib/utils/crypto.ts` hashing utilities

[DeFiVC]

What: Add cryptographic utilities for hashing, signing, and verification.

Why: Some operations need client-side hashing (e.g., request signing, data integrity). A utility module centralizes these.

Scope: Add functions for: sha256, hmacSign, verifySignature. Use Web Crypto API.

Acceptance criteria:

• Hashing works correctly
• Signing works
• Verification works
• Web Crypto API is used

Technical context: src/lib/utils/crypto.ts — crypto utils. src/lib/stellar/wallet.ts — signing.

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