Implement shared XDR sign-and-submit flow via `POST /stellar/send-transaction`

[grantfox-oss]

Context

Every write endpoint in the Trustless Work API (deploy, fund, change-milestone-status, approve-milestones, release-funds, update, dispute, resolve-dispute) returns an unsignedXdr that must be signed by the user's wallet and then submitted through this shared endpoint.

Task

Build one reusable helper/hook that:

1. Takes an unsignedXdr and requests a signature from the connected Stellar wallet.
2. Calls POST /stellar/send-transaction with { signedXdr }.
3. Returns txHash, ledger, and (for deploys) contractId + escrow to the caller.

All the other escrow-action issues (03–10) should build on top of this helper instead of duplicating sign/submit logic.

Acceptance criteria

• Single shared implementation, not duplicated per action.
• Handles wallet-rejection and network-failure states with clear error surfaces to the caller.

[GBOYEE]

Hi — every write endpoint returning an unsignedXdr currently duplicates sign-and-submit logic, which is error-prone. I can build one reusable hook that takes an unsignedXdr, requests the wallet signature, and submits via POST /stellar/send-transaction, then swap the duplicate call sites over to it. Happy to take this.