Introduced a random salt to key derivation for parent seed encryption to prevent rainbow table attacks. The salt is now stored in the `parent_seed.enc` file header (V3 format).
* Modified `src/utils/key_derivation.py` to accept optional `salt` argument.
* Modified `src/seedpass/core/encryption.py` to read/write salt in `parent_seed.enc` using V3 format (V3:salt:nonce:ciphertext).
* Modified `src/seedpass/core/manager.py` to generate salt for new seeds and read salt for existing ones.
* Added auto-upgrade logic for legacy (no salt) seeds upon access.
* Updated tests to verify salt usage and backward compatibility.
Co-authored-by: PR0M3TH3AN <[email protected]>
