Add reward claim tests and update TODO

[Achievers-sketch]

Add unit tests covering reward claim scenarios: test_successful_reward_claim (validates escrow, payout, and final balances/statuses), test_double_claim_prevention (expects Contract Error #6 on second approval), test_unauthorized_claim_rejection (expects Contract Error #3 for non-poster approval), and test_contract_balance_validation (verifies escrow, payout, and cancel refund behavior). Update TODO.md to replace the previous deployment task with a checklist for implementing and running these tests.

What

Why

Closes #30

How

Testing

Breaking Changes

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Checklist

General

• Branch is up to date with main
• PR title follows Conventional Commits format (e.g. feat: add reputation system)
• PR description is complete with issue reference (Closes #<issue-number>)
• One logical change per PR (no unrelated changes bundled in)

Testing

• Tests added or updated to cover the changes
• All tests pass (cargo test for contracts, npm run build for frontend)
• No compiler warnings (cargo clippy -- -D warnings)

Documentation

• Public API changes are documented with /// doc comments (Rust) or JSDoc (TypeScript)
• CONTRIBUTING.md, README.md, or other docs updated if needed
• CONTRACT_API.md updated if contract interface changed

Code Quality

• Code follows the project style (cargo fmt / ESLint)
• No any types introduced in TypeScript
• No hardcoded secrets, keys, or environment-specific values

Smart Contract (if applicable)

• require_auth() called on all state-changing operations
• Events emitted for all state changes
• Storage keys don't collide with existing keys
• Deadline and input validation is complete
• No unintended token transfer paths introduced

Frontend (if applicable)

• Components are under 150 lines
• UI is responsive and accessible
• Screenshots or recordings included for visual changes