feat: PGP signing and encryption

[andrinoff]

What?

Adds comprehensive PGP/OpenPGP support alongside the existing S/MIME implementation, using github.com/emersion/go-pgpmail (RFC 3156 compliant PGP/MIME).

Outgoing emails:

• Sign emails with PGP detached signatures (multipart/signed)
• Encrypt emails for recipients with known public keys (multipart/encrypted)
• Sender's own public key automatically included so sent mail remains readable

Incoming emails:

• Decrypt PGP-encrypted messages using the account's private key
• Verify PGP detached signatures
• Status badges in email view: [PGP: 🔒 Encrypted], [PGP: ✅ Verified], [PGP: ⚠️ Unverified]

Settings & configuration:

• Per-account PGP key paths (public + private), sign-by-default, and encrypt-by-default toggles
• Integrated into the existing crypto config screen alongside S/MIME
• Recipient public keys stored in ~/.config/matcha/pgp/<email>.asc
• Supports both ASCII-armored (.asc) and binary (.gpg) key formats

Composer:

• New "Encrypt Email (PGP)" toggle in the compose view
• PGP signing driven by account setting (sign-by-default)
• S/MIME and PGP are mutually exclusive per message

Why?

Closes #386

[andrinoff]

Know issues:

• Signatures are not verified
• For Yubikey, when Matcha expects you to touch the hardware key, there is nothing that says you have to (need to implement the "Touch the key to continue"
• The PIN is kept in OS keyring, questionable security