fix(backend): double XDR parse in /api/freeze/submit

[omolobamoyinoluwa-max]

Summary

/api/freeze/submit calls TransactionBuilder.fromXDR twice on the same signedXdr — once to extract adminKey and once to submit. This is redundant.

Fix

Parse once, reuse the result for both the source extraction and submission.

Acceptance criteria

• signedXdr is parsed exactly once
• Behaviour is unchanged