Implement minimum donation amount enforcement

[ayshadogo]

User Story:
As a developer, I want minimum donation amounts enforced on-chain so that dust donations don't pollute the donor list.
Acceptance Criteria:

• min_donation_amount: i128 stored in CampaignData
• Set during initialize
• Panics with Error::DonationTooSmall if amount < minimum
• Minimum set to 0 to disable (opt-in)

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[BestBisong]

@BestBisong has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would like to work on this issue

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[YaronZaki]

@YaronZaki has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello, please assign me this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @YaronZaki to this issue.

[drips-wave]

Congratulations, @YaronZaki! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @YaronZaki: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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