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and this machine's device key, with the account's row withdrawn when the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.

tests/DodoSSH.Client.App.Layout.Tests/LayoutHarness.cs

@@ -55,6 +55,20 @@ internal static class LayoutHarness
     /// <inheritdoc cref="TitleBarHeight" />
     internal const double StatusBarHeight = 24;
 
+    /// <summary>
+    /// What a setup card leaves its contents: its maximum width, less the padding on both sides.
+    /// </summary>
+    /// <remarks>
+    /// From <c>Border.card</c> in <c>App.axaml</c> — <c>MaxWidth</c> 520 and <c>Padding</c> 24 — because the
+    /// cards themselves live inside <c>MainWindow.axaml</c>, which cannot be laid out here at all. Measuring
+    /// a card's contents at the size the card gives them is the closest this harness can get to the unlock
+    /// screen, and it is the half that has something to blow: the frame is fixed and the contents are not.
+    /// </remarks>
+    internal const double CardContentWidth = 520 - (2 * 24);
+
+    /// <inheritdoc cref="CardContentWidth" />
+    internal static double CardContentHeight => ScreenHeight - (2 * 24);
+
     /// <summary>The height a screen actually gets at the window's minimum.</summary>
     internal static double ScreenHeight => MinimumHeight - TitleBarHeight - StatusBarHeight;