feat: implement escrow funding and transaction handling for single re…

[ryzen-xp]

📝 Pull Request Title

Implement escrow funding via single-release v2 endpoint

🛠️ Issue

• Closes Implement escrow funding via POST /escrow/single-release/v2/fund #308

📖 Description

• Adds escrow funding after deployment using POST /escrow/single-release/v2/fund.
• Requests the unsigned XDR, signs it with the seller’s wallet, and submits it through /stellar/send-transaction.
• Funds are marked locked only after transaction submission succeeds.

✅ Changes made

• Added typed Trustless Work funding and transaction APIs.
• Added server-side funding and Stellar transaction proxy routes.
• Added client funding workflow with status tracking.
• Handles insufficient balance, rejected signatures, missing XDR, and submission failures.
• Added a safe .env.example with TRUSTLESS_WORK_API_KEY.

🖼️ Media (screenshots/videos)

• Not applicable; this change adds API and transaction workflow logic.

📜 Additional Notes

• Call fundDeployedEscrow(...) immediately after a successful escrow deployment.
• Validation passed with ESLint, TypeScript, production build, and git diff --check.