[feat] Implement WebAuthn PRF for Server-Side Encryption at Rest

[secureprivate]

Current Architecture:
The server-side SQLite mailbox currently utilizes a key derived from an IMAP password for encryption at rest. WebAuthn PRF is deployed for the client-side App Lock feature, but the server-side storage requires password transmission and retention in active memory for database decryption during web or IMAP sessions.

Proposed feature:
Integrate the WebAuthn PRF extension to derive the primary decryption key for the server-side SQLite mailboxes, enabling direct database unlocking via hardware authenticators for webmail access.

Technical rationale:

• Zero-Knowledge Web Sessions: Deriving the key locally via PRF allows the web application to unlock the server database without transmitting or storing a plaintext password in the server's RAM.
• Phishing Immunity: The hardware authenticator generates the key locally. Phishing domains cannot extract the underlying password or private key.
• Cryptographic Domain Binding: The PRF extension binds the cryptographic salt to the specific relying party domain. Requests from spoofed domains generate mathematically invalid keys.
• Separation of Concerns: Authentication (proving possession via cryptographic signature) is isolated from decryption (deriving the database key via PRF).

Implementation:

1. Enable PRF key derivation to serve as the master key for the server-side ChaCha20-Poly1305 encryption.
2. Restrict standard password-derived keys strictly as a fallback mechanism for legacy clients connecting via IMAP/POP3/SMTP protocols.
3. Ensure the server only receives the derived key required to unlock the SQLite instance during active web sessions, discarding it upon session termination.