feat(contract): cancellation refund math

[walterthesmart]

Implement refund calculation for cancelled missions.

Changes

• Added cancel_mission(env: Env, mission_id: u64) function to QuidStoreContract.
• Implemented logic to calculate unspent funds: (max_participants - participants_count) * reward_amount.
• Refund logic triggers if refund_amount > 0.
• Transfers tokens from contract to mission owner using token::Client.
• Updates mission status to Cancelled.
• Emits mission_cancelled event.
• Prevents double refund by checking if mission is already Closed/Cancelled.

Fixes

Closes #38

Test

Added test_cancel_mission_full_refund to verify full refund on cancellation.
Added test_cancel_already_cancelled_mission_fails to verify idempotent safety.

[walterthesmart]

@Kaylahray, Kindly look into this fix and merge accordingly.

[Kaylahray]

Thank you for completing this issue.