chores

[Cedarich]

• Defines StakeInfo { amount, lock_timestamp } and DataKey::{Admin, Token, UserStake(Address)}
• Implements stake to transfer USDC into the vault, accumulate stake, and reset lock timestamp
• Implements unstake to enforce 7-day lock-up, transfer full stake back, and clear storage
• Adds tests for staking, lock enforcement, and post-withdrawal cleanup

closes #45
closes #47
closes #46

[drips-wave]

@Cedarich Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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