feat(contracts): TokenTransfer Soroban contract with testnet deploy script

[codebestia]

Summary

• New token_transfer contract in contracts/contracts/token_transfer/
• Wraps any SEP-41 token (configured at init time) — does not mint its own token
• initialize(admin, token_contract) — one-time setup; panics on double-init
• transfer(from, to, amount, memo) — requires from.require_auth(), delegates to the SEP-41 token, emits a TransferEvent with an optional opaque memo bytes field (for linking to chat message IDs)
• balance(address) — proxies balance query to the underlying token
• set_token_contract(new_token) — admin-only token migration escape hatch
• 6 unit tests using a mock SEP-41 token: init, double-init panic, transfer, balance, zero-amount panic, sender auth check — all passing
• WASM builds to 9.6K at target/wasm32-unknown-unknown/release/token_transfer.wasm
• contracts/scripts/deploy_token_transfer.sh — builds, uploads WASM, deploys instance, and calls initialize on testnet via the stellar CLI
• Added TOKEN_TRANSFER_CONTRACT_ID to .env.example