fix: add end-to-end submission idempotency (#213)

[pharwasz]

Summary

• Persist a stable operation ID for Stellar sends and carry it through offline queue review and broadcast retries.
• Serialize submissions across tabs and reuse the original signed envelope after an uncertain response.
• Add unit and end-to-end coverage for concurrent broadcasts and delayed retries.

Verification

• 45 unit tests passed.
• 5 offline-queue Chromium E2E tests passed.
• Production build, formatting check, and dependency audit gate passed.

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closes #213

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[truthixify]

Submission IDs bind to a signed envelope, retries reuse that envelope, and focused tests plus the production build pass. Approved.