Comprehensive contract error enum

[ayshadogo]

User Story:
As a developer integrating with the contract, I want every failure mode mapped to a specific typed error so that callers can handle conditions programmatically instead of matching on panic strings.

Acceptance Criteria:

• #[contracterror] Error enum covering: AlreadyInitialized, NotInitialized, Unauthorized, CampaignEnded, CampaignNotActive, AssetNotAccepted, DonationTooSmall, MilestoneNotFound, MilestoneNotUnlocked, PreviousMilestoneNotReleased, CannotCancelWithFunds, RefundWindowClosed, InvalidGoalAmount, InvalidEndTime, InvalidMilestones, InsufficientContractBalance, Overflow, Reentrant, ContractFrozen
• Every variant documented with when it's raised and whether it's recoverable (bad input, retry-able) or terminal (indicates a bug/attack)
• Audit pass confirming no function in the contract panics with an unlabelled string where a typed error applies

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[Chigybillionz]

I can take this up and begin immediately

[nafsonig]

let me handle this issue as soon as possible

[grantfox-oss]

🦊 GrantFox — @nafsonig 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 #41)
2. Your PR will be reviewed by the Dfunder maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@nafsonig's PR #52 was approved and merged by @ayshadogo.

🏆 @nafsonig: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,062 total points). Track your full progress on GrantFox.

👏 Great work, @nafsonig! Keep contributing to Dfunder.