`ContentCipher` makes a person's data keys, sealed to their account and
signed by it, opens them, and seals and opens a row's content with them for
the row it is stored in (`SodiumContentCipher`, on the isolate it is used
on, as rows are read and written one by one; `FakeContentCipher` for
tests). A sealed payload holds the signature, then the ciphertext.
`AccountCrypto.newDeviceKeys` makes the key pairs an anonymous person's
device encrypts their private studysets with (ADR 0017, point 9, not built
before); setting a password keeps them as the account's
(`keepingKeys` with a new phrase), so nothing they made private is lost.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>