Add Encryption architecture page Covers key hierarchy (Argon2id KEK -> master key -> per-note DEK), keychain sync design, HMAC sync hash, session lifecycle, multi-device semantics (multi-keychain unlock, per-note passphrase, keychain adoption), passphrase change / recovery rotation, and limitations. Generated with [Devin](https://devin.ai) Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>