Task: Add optional password protection / encryption for sensitive snippets

[tristanbudd]

Description:
Implement a security feature enabling users to optional lock pastes with a password, requiring validation before rendering.

Tasks:

• Update frontend UI with a toggle and input field for optional password protection.
• Modify backend paste creation endpoint to accept a password/salt or support client-side/server-side encryption.
• Create authentication gate/modal in frontend to prompt for password before viewing protected pastes.
• Enforce backend-side protection so raw API requests also validate the password.
• Make an setting on config file to enable / disable this feature.