feat: emergency circuit breaker and timelock governance (#27)

[Maximum-Prosper]

Description

Implements the emergency circuit breaker mechanism, emergency token rescue, and 24-hour timelock governance for admin changes and WASM contract upgrades in the yield_vault Soroban smart contract.

Closes #27

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Verification Commands

• npm run lint / npm run test (Frontend/Backend)
• cargo fmt / cargo clippy / cargo test (Smart Contracts)

Contract Security (if contracts/ changed)

• Storage schema changes documented or migration provided (DataKey::PendingUpgrade)
• All entry points have authorization checks (Self::require_admin)
• Arithmetic uses checked operations
• New entry points have unit tests including unauthorized callers
• No new admin roles without governance proposal

UI Snapshot Checklist

• No visual changes (This PR modifies smart contract backend logic in contracts/yield_vault)

Additional Notes

• emergency_pause() halts all deposits, rebalances, harvests, and flash loans immediately while user withdrawals remain open.
• rescue_funds() enables rescuing tokens directly to a target address and updates TotalAssets.
• set_admin enforces a 24-hour timelock delay between proposal and execution.
• propose_upgrade and commit_upgrade enforce a 24-hour timelock delay before executing contract WASM code upgrades.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

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