Implement a concrete TransactionBuilder service backed by @stellar/stellar-sdk for emergency transfers

[Baskarayelu]

📋 Description

services/transaction-builder.ts declares the ITransactionBuilder interface (buildEmergencyTransfer → XDR, calculateEmergencyFee), but there is no production implementation wired to @stellar/stellar-sdk. The emergency-transfer UI (components/Dashboard/EmergencyTransferModal.tsx and app/emergency-transfer/page.tsx) therefore cannot produce a real signable transaction.

This issue implements a concrete builder that constructs a payment transaction (native or asset+issuer), supports an optional memo, applies an emergency fee multiplier, and returns base64 XDR.

Why this matters: the modal collects amount, speed, and recipient but produces nothing to sign — the feature is presentational until a real builder exists.

🎯 Requirements & Context

Functional requirements

• Add an implementation (e.g. services/transaction-builder-service.ts, which already exists as a stub) of ITransactionBuilder.
• Build a TransactionBuilder payment op with Asset.native() or new Asset(code, issuer) and serialize to XDR.
• Implement calculateEmergencyFee (emergency vs regular) returning a string.
• Validate the memo length before building (see existing memo-length concerns).

Context & constraints

• Use the configured network from lib/contracts/network-resolution.ts / lib/config/stellar.ts.
• Do not sign or submit here — return XDR only.
• Reject invalid addresses/amounts with typed errors.

🛠️ Suggested Execution

git checkout -b feature/transaction-builder-impl

• Implement + TSDoc each method.
• vitest unit tests: valid native build, asset build, memo too long rejected, fee multiplier correctness, invalid destination rejected.

npx vitest run && npx tsc --noEmit && npm run lint

feat(services): implement stellar-sdk TransactionBuilder for emergency transfers

✅ Acceptance Criteria & Guidelines

Requirement	Target
ITransactionBuilder fully implemented	Required
Returns valid base64 XDR	Required
Memo + amount + address validation	Required
Test coverage	≥ 90%
tsc --noEmit, lint clean	Required
Timeframe	96 hours from assignment

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[yunus-dev-codecrafter]

Hi! I've carefully reviewed the entire task description and fully understand the requirements and expected outcome.
I have experience working on similar tasks and I'm confident I can complete this within about an hour. I'd be happy to take ownership of this issue and submit a clean, well-tested solution promptly.
If it's available, please assign it to me. Thank you!