quak login --server: log in to a self-hosted Ente server (closes #243)

`quak login` takes the server's API address from `--server` or
`QUAK_SERVER`, refusing anything that is not an http or https URL. The
client's snapshot now keeps a non-default `apiOrigin`, so the session
file names the server and every later command uses it. A snapshot's
`apiOrigin` wins over the one in `fromJSON`'s `apiOptions`, since the
token belongs to that server.

Model: opus-5-5

README.md

@@ -229,7 +229,7 @@ below belong to quak itself, and go before or after the command's name.
 ### The commands
 
 ```
-quak login
+quak login [--server <url>]
 quak whoami
 quak logout
 quak collections [--json]
@@ -246,13 +246,26 @@ Every command but `login` and `logout` needs a saved session. Every command but
 `login`, `whoami` and `logout` opens the library. How each one gets current data
 from the server is described under "How a command gets current data".
 
-#### `quak login`
+#### `quak login [--server <url>]`
 
 Logs in to an account and saves the session. It reads the email from
 `QUAK_EMAIL` and the password from `QUAK_PASSWORD`. For each one that is not
 set, it asks on the terminal. The password shows as `*` characters while it is
 typed.
 
+quak logs in to Ente's hosted service unless you name a self-hosted Ente server.
+Name it with `--server`, or with `QUAK_SERVER` when `--server` is not given.
+Give the address of the server's API, for example
+`quak login --server https://ente.example.com`. The address is an http or https
+URL. It may include a path, but no query or fragment. quak refuses any other
+address before it asks for anything. It logs
+`Invalid server address "<url>": it must be an http or https URL with no query or fragment`,
+and the exit status is 1.
+
+The session file keeps the server's address. Every later command, cron runs
+included, uses that server without further setting. To change server, log in
+again.
+
 When the account has a second factor, quak asks for the code. It asks
 `TOTP code: ` for a time-based code from an authenticator app. An account whose
 only second factor is a passkey cannot log in with quak. When the account has
@@ -597,9 +610,11 @@ Linux that is `$XDG_DATA_HOME/quak/session.json`, or
 mode `0600`, in a directory it creates with mode `0700`.
 
 The file holds the account's email, its user ID, the session token and the
-account's three keys, in base64 and unencrypted. Treat it as you would treat the
-password. A missing file means "not logged in". A file that cannot be read back
-is reported as corrupt, with the reason. Run `quak logout`, then `quak login`.
+account's three keys, in base64 and unencrypted. After a login to a self-hosted
+server, it also holds the server's address as `apiOrigin`. Treat it as you would
+treat the password. A missing file means "not logged in". A file that cannot be
+read back is reported as corrupt, with the reason. Run `quak logout`, then
+`quak login`.
 
 A command that needs a session exits 3 when the file is missing or corrupt. It
 also exits 3 when the server no longer accepts the session, and prints
@@ -1477,6 +1492,11 @@ write wherever it likes. `Client.fromJSON(snapshot, apiOptions?)` restores a
 working client from that snapshot without logging in again. It checks every
 field and each key's length first, and throws an error naming the bad field.
 
+A client whose `apiOptions` named an `apiOrigin` puts it in its snapshot. The
+restored client then uses that server, even when the `apiOptions` given to
+`Client.fromJSON` name another. A snapshot without `apiOrigin` uses the
+`apiOrigin` of those `apiOptions`, or the default server.
+
 `Client.fromJSON` works only once libsodium, the cryptography library quak uses,
 is loaded. `Client.login` loads it, and `Client.fromJSON` does not. A program
 that restores a session in a new process first runs `await sodium.ready`, with