Replace hardcoded 'zQ3sh' with SECP256K1_PUBLIC_MULTIBASE_PREFIX constant

[Copilot]

Addresses review feedback on #47 to use the SECP256K1_PUBLIC_MULTIBASE_PREFIX constant instead of a hardcoded string for consistency.

Changes:

• Updated getRotationPublicKeyMultibase() to use the constant in both the validation check and error message

// Before
if (!multibase.startsWith('zQ3sh')) {
  throw new RotationKeyInputError(
    `Invalid rotation key format. Expected a Secp256k1 key starting with 'zQ3sh', got '${multibase.slice(0, 5)}...'`,
  );
}

// After
if (!multibase.startsWith(SECP256K1_PUBLIC_MULTIBASE_PREFIX)) {
  throw new RotationKeyInputError(
    `Invalid rotation key format. Expected a Secp256k1 key starting with '${SECP256K1_PUBLIC_MULTIBASE_PREFIX}', got '${multibase.slice(0, 5)}...'`,
  );
}

This aligns with line 313 which already uses the constant.

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