Let the recovery code be copied, and give the phone a clipboard to copy to

Both screens had made the code selectable and both said why: a person who cannot
get it out of the box photographs the screen, and a screenshot is a worse home
for it than a clipboard. This finishes that argument. Selecting 64 characters of
letter-spaced monospace with a thumb is the version of "possible" people give up
on halfway — and on the phone the screen blocks screenshots, so the honest
remaining options were retyping it or losing it.

It is the one secret this application deliberately offers to a clipboard, and the
contrast with the keychain's copy is the whole argument rather than an
inconsistency. There, copying the private half is refused outright, because
installing a key means pasting the public one and the private one has no business
leaving the vault. Here there is no better route: the code exists for one screen,
is stored nowhere, and has to reach a password manager. The clipboard is the
intended destination rather than a way round the design.

The sentence afterwards matters as much as the copy, and is asserted: a clipboard
is a staging post, this screen is the only place the code exists, and the next
thing copied replaces it. Somebody who copies and does nothing has not saved it.

The phone had no clipboard delegate at all — the desktop passed one and this head
passed null — so COPY PUBLIC KEY on the keychain answered "this machine has no
clipboard" on a device that plainly has one. Nothing about that was platform
shaped: Android has a clipboard and Avalonia surfaces it through the same
TopLevel. Wiring it fixes that copy too.

The test fixture built its shell without a clipboard, which modelled the bug
rather than the product, so it has one now and the public-key test asserts what
lands there instead of the refusal. The refusal keeps its own test, on a shell
built without one, because the view model reads the delegate's absence rather
than an empty result — and because a button that silently does nothing on this
screen is worse than one that refuses.

src/DodoSSH.Client.App/Views/MainWindow.axaml

@@ -297,8 +297,24 @@
                                      FontSize="17" Foreground="{StaticResource Accent}"
                                      TextWrapping="Wrap" />
               </Border>
-              <CheckBox IsChecked="{Binding RecoveryCodeWrittenDown}"
-                        Content="I have written it down somewhere safe" />
+              <!--
+                ◆ COPY, beside the tick rather than inside the box. The code is selectable and has been
+                since this card existed, on the argument that refusing to let somebody take it only pushes
+                them to a screenshot — this is that argument finished, because "select 64 characters of
+                letter-spaced monospace without missing one" is a thing people get wrong silently.
+
+                It is the one secret this application offers to the clipboard on purpose; see
+                MainWindowViewModel.CopyRecoveryCode for why that is right here and refused for a private
+                key, and for what the status line below says afterwards.
+              -->
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <Button Classes="ghost" Content="COPY CODE"
+                        Command="{Binding CopyRecoveryCodeCommand}"
+                        ToolTip.Tip="Puts the code on the clipboard. Paste it into your password manager now — it is shown once and the next thing you copy replaces it." />
+                <CheckBox IsChecked="{Binding RecoveryCodeWrittenDown}"
+                          Content="I have written it down somewhere safe"
+                          VerticalAlignment="Center" />
+              </StackPanel>
               <Button Classes="accent" Content="CONTINUE" Command="{Binding ConfirmRecoveryCodeCommand}"
                       HorizontalAlignment="Left" />
               <TextBlock Classes="hint" Text="{Binding StatusMessage}" />