SEP-45 Implementation

[jopmiddelkamp]

Implement SEP-45 (Stellar Web Authentication with Contract Accounts).

SEP-45 enables contract-based WebAuth. All three peer SDKs (Flutter, iOS, Java) have already shipped this — we are the only SDK without it.

SEP-45 builds on the existing SEP-10 (WebAuth) infrastructure. The SDK already supports Soroban authorization entries needed for contract account authentication.

Acceptance Criteria

• SEP-45 implemented with unit tests
• Closes the most visible feature gap versus all peer SDKs