fix(stellar): build real Horizon-sequenced payment tx in signRentPayment

[BWM0223]

Summary

Fixes #7 - signRentPayment builds a meaningless placeholder transaction

Root Cause

used with zero operations. The XDR encoded an empty tx with a fake sequence that can never be submitted. and were accepted but completely unused. and from .env were never read.

Fix

• Loads real account sequence from Horizon via
• Adds targeting
• Supports native XLM and custom assets via /
• Encodes as memo for contract-side agreement identification
• Throws early with clear error if is not configured
• Returns or throws if Freighter declines

Acceptance Criteria

• Signing path loads real account sequence from Horizon
• Transaction includes a real payment operation to the contract
• propertyId is encoded in memo
• NEXT_PUBLIC_HORIZON_URL and CONTRACT_ID are consumed from env