Give the phone a way to enrol the fingerprint it already unlocks with

The Android device key store, the biometric gate and the lock screen's UNLOCK
WITH FINGERPRINT button have all shipped since this head was written, and none
of them could ever run: that button appears only when a device key exists, and
nothing on the phone could create one. `CanUnlockWithDevice` was false on every
launch of every phone. This is the missing half.

**The offer is on PREFERENCES**, which held a PendingScreen until it had a
setting on it. It is there rather than beside the button it turns on because
registering needs an unlocked keychain and a reachable server — the vault has to
be open to seal the bundle, and the wrap has to reach the account or a phone
somebody has lost could never be revoked. Neither is true on the lock screen.

One card, and exactly one of its three blocks is ever drawn: the offer, the
withdrawal, or the sentence saying this phone has nowhere to keep a key. That is
`CanRegisterDevice` / `CanForgetDevice` / `HasNoDeviceKeyOption`, which are two
flags and not one and its negation for the reason written where they are set —
a phone with no screen lock and a phone already registered are both "cannot
register", and only the second has anything to take back. The withdrawal has no
confirmation, deliberately, and the sentence above it carries what the desktop
puts in a tooltip this head has no room for. `StatusMessage` is on the screen
because it is the only feedback this head has once the system's own dialogue has
gone.

**Two things would have been wrong in the feature the moment it worked.**

`Environment.MachineName` answers `localhost` on Android, and registering names
the device — so every phone would have arrived in the account's device list as
another identical row, on the very screen a lost handset is revoked from.
`PhoneEnvironment.DeviceName` was already written and never called; the shell
now takes it as an optional constructor argument that the desktop does not pass,
and it reaches enrollment, registration and every connection log entry. That was
gap §7 of docs/android-port.md, and it is now closed.

And the status line said "Waiting for Windows…" over an Android biometric
prompt. `GestureWait` picks the sentence from the platform rather than from a
head, unlike the device name beside it: a device name is a fact about one
handset only the head can read, and which dialogue appears is a fact about the
operating system this assembly is running on.

Two tests cover the seam — the injected name reaching the account, and the
default still being this machine's own name — and `FakeVaultServer` records what
each device called itself, because the name is the only part of a registration a
person ever reads. The gesture itself is unreachable from any test process, so
Phase 13 of docs/manual-checks.md carries five checks, including that enrolling
a new fingerprint in Android's own Settings destroys the key. That one is the
property that makes this a fast path rather than a weakening of the passphrase.

src/DodoSSH.Client.Android/Views/PhoneShell.axaml

@@ -174,10 +174,12 @@
         </Panel>
 
         <!--
-          Preferences keeps PendingScreen for its body — it is still a screen that states what is absent
-          rather than one with settings on it — and gains the header its siblings under MORE have, here
-          rather than inside PendingScreen: that control is shared with three states of getting in, none of
-          which has anywhere to go back to.
+          Preferences carries the header its siblings under MORE have, written here rather than inside the
+          screen: the screen is one of two that hang off this hub with a back arrow, and the arrow belongs
+          to the shell that knows where back is.
+
+          It held a PendingScreen until it had a setting on it. The one it gained is this phone's device
+          key — the fingerprint the lock screen offers and that nothing here could previously enrol.
         -->
         <Grid IsVisible="{Binding IsPreferencesShowing}" RowDefinitions="Auto,*">
           <Grid Grid.Row="0" ColumnDefinitions="Auto,*" Height="56" Margin="8,0">
@@ -185,9 +187,7 @@
                     CommandParameter="{x:Static vm:ShellScreen.More}" />
             <TextBlock Grid.Column="1" Classes="heading" Text="Preferences" Margin="4,0" />
           </Grid>
-          <views:PendingScreen Grid.Row="1"
-                               Heading="PREFERENCES"
-                               Detail="Registering this phone's device key, and signing out, are on the unlock screen for now. Importing an ~/.ssh/config has no meaning here — scoped storage means there is no such file to read." />
+          <views:PreferencesScreen Grid.Row="1" />
         </Grid>
       </Panel>