Stay signed in, come back online by itself, and let a machine be given up

Three things a machine that has been set up could not do. Unlock now takes
Enter, which is the gesture everybody makes after typing a password and which
did nothing until they found the button.

Signing in survives a relaunch. The refresh token is kept in the local cache,
sealed under the vault's own cache key, so a later launch resumes the session
through the refresh grant with no browser and nobody present — and because it
is sealed under that key, only an unlocked vault can resume it. A locked
client therefore cannot reach the server at all, which is a consequence worth
stating rather than working around; docs/crypto.md §3.2 records it. Every sync
pass asks the shell for a connection rather than reading one captured at
unlock, so a laptop that unlocked on a train is online within a minute of
finding a network, with nothing pressed. Unlocking itself still never waits on
a socket.

Signing out empties this machine: the profile, the cached items, the outbox
and this machine's device key, with the account's row withdrawn when the
server can be reached. It asks first and says what it costs — the outbox count
when the vault is open, an admission that it cannot be counted when it is not,
and the shells that keep running either way. The vault is on the server and is
untouched, which is what makes the same button the only honest answer to a
forgotten passphrase, so it is on the unlock screen as well as in preferences.
It cannot end the session at the identity provider, and says so.

Two defects surfaced on the way. The synchronisation pass that runs when the
vault opens never ran at all: the loop is started from inside the unlock
command, so the busy flag it yields to was raised by that command — the first
sync was a minute late on every launch. And signing in from preferences while
unlocked threw an unlock screen over an open vault whose keys were still in
memory.

The unlock card and the new confirmation live in their own controls because
MainWindow cannot be laid out headless, so markup left inside it is markup no
test can measure; both are now measured at the window's minimum size in the
shapes that grow. What is still unverified is the composed window itself.

README.md

@@ -145,6 +145,21 @@ the next sync. If a server is legitimately rebuilt and offers a new key, the con
 with no way to continue from the warning — edit the host and choose **Forget host key**, which is deliberately
 somewhere you have to go on purpose.
 
+**Signing in once is enough.** The refresh token is kept in the local cache, sealed under the vault's own
+key, so a later launch resumes the session itself and no browser opens — and because it is sealed under that
+key, resuming can only happen *after* the vault is unlocked. A machine that unlocks with no network keeps
+trying: every synchronisation pass asks for a connection, so a laptop opened on a train is online again
+within a minute of finding a network, with nothing pressed. Unlock takes **Enter** in the passphrase box,
+and nothing about unlocking ever waits on the network.
+
+**Signing out** is under Preferences → *Account*, and again on the unlock screen, where it is the only
+answer to a forgotten passphrase — nothing can recover one. It asks first, and says what it costs: it
+empties this machine's cache (the profile, the cached items, and anything still queued to be sent) and
+withdraws this machine's device key from the account. The vault itself is on the server and is untouched, so
+signing in again brings it all back; the count in the confirmation is the one thing that exists nowhere
+else. Your session at the identity provider is *not* ended — DodoSSH has no way to end it — so on a machine
+that is not yours, sign out there too.
+
 Two of M1's known gaps are visible immediately, so they are worth expecting rather than diagnosing: password
 authentication asks for the password every time, because nothing in the interface can create a vault
 credential yet (they do sync — there is just no editor for one); and unlock asks for the passphrase on every