Add Username/Password Authentication

[she3o]

Feature Overview

Only authorized users should be able to sync their Zotero selection.

Requirements

• Allow configuration and storage of usernames and passwords safely (So not through environment variables or config files).
• Provide clear error messages for authentication failures.

Acceptance Criteria

• Users must provide valid credentials to access the WebDAV server.
• Unauthorized access attempts are logged and denied.

Implementation Ideas

• Secure storage: Hash credentials if they are stored on the server.