implement Multi-Asset Ledger Timeout (Time-Bounds) Enforcement

[CollinsC1O]

Description:

Implemented and verified Multi-Asset Ledger Timeout (Time-Bounds) Enforcement

• Enforced strict 15s transaction time-bounds for single price updates, batched multi-asset updates, and multi-signed updates in stellarService.ts
• Added a submit-time guard so any transaction envelope without strict time-bounds is rejected before signing/submission.
• Added a local timeout listener that tracks pending submissions, marks expired ones, invalidates the local sequence assignment, and retries immediately with a fresh assignment.

Closes #283

[drips-wave]

@CollinsC1O Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits