lib/e2ee was 14% covered and fails silently in every way it can break. Real WebCrypto and an
in-memory IndexedDB: a message decrypts for its partner and nobody else, every message gets its own
IV, a new device unlocks with the passphrase and refuses a wrong one, the key survives a reload
until reset, a partner's rotated key is used only after forgetConv, and the key check never shares
the wrap IV (GCM nonce reuse). Mutations seen red: the shared IV, a no-op forgetConv, no persist.