Implement campaign deadline enforcement in `donate`

[ayshadogo]

User Story:
As a developer, I want donations rejected after the campaign deadline so that the contract enforces time-bound fundraising.
Acceptance Criteria:

• env.ledger().timestamp() > campaign.end_time check in donate
• Panics with Error::CampaignEnded
• Deadline checked on every donation
• Atomic check-and-transfer to prevent race conditions

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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.

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