Refactor keychain password retrieval and optimize encryption/decryption logic

Commit Details 🚀

SHA-1: e5eb71ac63d7e5a92b184d28b7675d86edf22a70
MD5: 8a421390b604a771e76bed3e51469c09

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Changes ✨

• 🔐 Secure Password Retrieval:
  Modified the logic to fallback between "Signal Key" and "Signal" from Keychain.

• 🔒 Encryption/Decryption Optimization:
  Updated encrypt_string and decrypt_string implementations to use a fixed IV per macOS OS_Crypt requirements.

• 🛠️ Improvements:
  Enhanced code readability, documentation, and error handling.

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