feat: add freelance milestone escrow demo app

[collinsezedike]

Summary

• Adds demos/freelance-escrow: a real freelance/gig milestone-payment app built on Tholos as its settlement layer, not a UI of buttons calling contract functions directly.
• Wallet connect (Freighter), jobs/milestones list, post-job flow, and a role switcher (freelancer/client/resolver) to exercise all sides of a dispute from one browser.
• Full Tholos wiring: assert_outcome, dispute, resolve, finalize, get_assertion_state, following the end-user-as-asserter pattern in docs/src/INTEGRATION.md.
• Wired against a live Tholos testnet deployment by default, so milestone actions are real signed transactions.

Closes #57

Test plan

• pnpm build and pnpm lint pass clean in demos/freelance-escrow
• cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test unaffected (no contract changes)
• Manually exercised the full flow against the deployed testnet instance: post a job, mark a milestone done, dispute, resolve, finalize