Refund eligibility logic and `request_refund`

[ayshadogo]

User Story:
As a donor, I want to request a refund when a campaign is cancelled or fails, under clearly defined on-chain rules, so that my funds aren't stranded.

Acceptance Criteria:

• Eligibility, exposed as a view function: campaign Cancelled, OR Ended with zero milestones released AND within the refund window; documented in contract comments
• fn request_refund(env, donor): donor.require_auth(), re-validates eligibility server-side, checks non-zero donor balance, transfers the donor's contributed amount back, clears the donor's record, emits refund_issued
• Tests: refund from a cancelled campaign, refund attempt on an active campaign (panics), double-refund attempt

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[a-malik-gh]

I can take this on by implementing the on-chain refund eligibility rules, secure refund flow, state cleanup, event emission, and tests covering cancelled, active, and double-refund scenarios.

[grantfox-oss]

🦊 GrantFox — @a-malik-gh has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #37)
2. Your PR will be reviewed by the Dfunder maintainers

Good luck! Track your progress on GrantFox.