carddav: fix contact encryption key selection (user key vs address key)

[haurucknde]

Problem

CardDAV contact creation/updates fail with openpgp: incorrect key or produce contacts
that show "The decryption of the encrypted content failed" in Proton's web UI.

Fixes #157, fixes #288.

Root Cause

Unlock() returned only address keys in the key ring. However, Proton's web client
encrypts and signs contacts using the user key (the master account key), not the
address key. On accounts where the user key and address key differ — which is the case
for all modern Proton accounts — contacts written by hydroxide cannot be decrypted by
the Proton client.

Fix

• Unlock() now includes user keys in the returned key ring (user keys first, then
  address keys), and tracks the primary user key ID separately.
• The CardDAV handler encrypts and signs contact data using the primary user key,
  matching the behaviour of Proton's own clients.

Testing

Verified on a modern Proton account (separate user key and address key):

• PUT a vCard with FN, EMAIL, NOTE, CATEGORIES → HTTP 201
• Contact opens in Proton web UI without any decryption warning
• All fields (including encrypted detail fields) render correctly
• CalDAV (IMAP/SMTP) continues to work normally after this change