feat: add depositWithPermit for single-tx EIP-2612 deposits

[Tranquil-Flow]

Summary

Adds depositWithPermit() enabling single-transaction deposits using EIP-2612 permit signatures — no separate approve() tx needed.

Changes

• SafetyNet.sol: New depositWithPermit(id, value, deadline, v, r, s) function
  • Imports IERC20Permit
  • Detects onboarding vs regular deposit to set correct permit amount
  • Calls IERC20Permit.permit() then delegates to existing _deposit()
• ISafetyNet.sol: Interface declaration with NatSpec
• MockERC20Permit.sol: New test mock (ERC20 + ERC20Permit)

Testing

3 new tests: onboarding via permit, regular epoch deposit via permit, expired deadline revert. All 100 tests pass.

Note: If #46 (_msgSender) lands first, msg.sender in the permit function should be updated to _msgSender().

Closes #32