Let the host editor make the credential it is about to bind

The authentication picker has listed saved credentials since they existed, but
making one meant leaving a half-typed host for the keychain screen and coming
back to find it gone. On the phone it was worse than a detour: that head has no
credential editor at all, so it could bind a host to a credential and never
produce one. + NEW CREDENTIAL opens a card under the picker — name, optional
username, password, notes — and ADD writes it and binds the host in one step.

A button beside the picker rather than an entry inside it. Every row of that
list is a binding a host can have, and "make a new one" is an action: as an
entry it would sit in the box afterwards describing a state no host can be in,
and cancelling the form would leave the picker showing it.

It carries its own five fields rather than reusing the keychain editor's, and
that is the load-bearing part. IsEditingCredential is what AVaultEditorIsInTheWay
asks about, so sharing it would have made the whole Vault screen refuse to open
an editor while this card sat open on the Hosts screen, with a status line
naming a form the user cannot see on a screen they are not looking at — the
exact failure that guard was split in two to end. A test pins it.

It writes to the keychain immediately, unlike every other field in this editor,
because a credential is a shared item with an id and a host can only name an id
that exists. The consequence is honest rather than hidden and the hint says so:
a credential added this way outlives a cancelled host edit. What was still being
typed does not — every path that closes the host editor clears the form, and one
of those fields is a password.

The binding is written before the reload rather than after it. RefreshOpenEditors
rebuilds this picker and then restores it from the editor's own selection, so
setting it first is what survives the pass, and by the time it is read
ReloadCredentialsAsync has put the matching entry in the list to land on.

A name already taken is duplicated, not reused, and that is a deliberate parting
from the new-tag box six lines further down which offers the existing tag
instead. Two tags called "staging" are one intention spelled twice; two
credentials called "root" are two different passwords, and quietly binding the
host to whichever was there already would authenticate it as an account nobody
chose. A duplicate label in the picker is the smaller problem.

Into editingHostVaultId, so the credential lands wherever the host is being
sealed and everybody who can read the host can read what it authenticates with.
Stricter than the tag path — which files into the active vault and is recorded
as a gap in docs/design-import-gaps.md — and it can be, because this picker
lists credentials from every readable vault rather than one.

Five flow tests cover the bind-through-reload path, the cancel semantics on both
the saved credential and the abandoned one, the cross-screen guard, the
duplicate name and the empty-password refusal. The layout test is separate and
necessary: the card is collapsed until somebody presses the button, so a harness
driven by the default state draws none of it, and TheHostDrawerFitsWithTheHostEditorOpen
would have gone on passing over a card that blew the column. 1,854 tests, none
failing.

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

@@ -584,6 +584,56 @@
                   </ComboBox.ItemTemplate>
                 </ComboBox>
 
+                <!--
+                  Making a credential without leaving the host. The moment one is wanted is this one: somebody
+                  is deciding how a host authenticates and finds the password is not in the keychain yet, and
+                  sending them to the other screen to add it would lose the half-typed host they are standing
+                  in. Same argument as the new-tag box further down, same immediate write, same honest
+                  consequence — the credential stays if this editor is cancelled, because a host can only name
+                  an id that exists.
+
+                  A button beside the picker rather than an entry inside it. Every row of that list is a
+                  binding the host can have; "make a new one" is an action, and as an entry it would sit in the
+                  box afterwards describing a state no host can be in.
+                -->
+                <Button Classes="ghost" Content="+ NEW CREDENTIAL" HorizontalAlignment="Left"
+                        FontSize="10.5" Height="28" Padding="10,0"
+                        IsVisible="{Binding !IsAddingEditorCredential}"
+                        Command="{Binding BeginEditorCredentialCommand}"
+                        ToolTip.Tip="Adds a credential to the keychain and binds this host to it" />
+
+                <Border CornerRadius="12" Background="{StaticResource Field}"
+                        BorderBrush="{StaticResource Border}" BorderThickness="1" Padding="12"
+                        IsVisible="{Binding IsAddingEditorCredential}">
+                  <StackPanel Spacing="6">
+                    <TextBlock Classes="label" Text="NEW CREDENTIAL" FontSize="10" />
+                    <TextBox Text="{Binding EditorNewCredentialLabel}" PlaceholderText="name" Height="36" />
+                    <!--
+                      Optional, and what makes a credential worth being its own item: one account on twenty
+                      machines is rotated in one place. Left blank, this host's own username is used.
+                    -->
+                    <TextBox Text="{Binding EditorNewCredentialUsername}" Height="36"
+                             PlaceholderText="username (blank: use this host's own)" />
+                    <!-- Masked, on the reasoning the keychain's own password box carries. -->
+                    <TextBox Text="{Binding EditorNewCredentialPassword}" PlaceholderText="password"
+                             PasswordChar="•" Height="36">
+                      <TextBox.KeyBindings>
+                        <KeyBinding Gesture="Enter" Command="{Binding AddEditorCredentialCommand}" />
+                      </TextBox.KeyBindings>
+                    </TextBox>
+                    <TextBox Text="{Binding EditorNewCredentialNotes}" PlaceholderText="notes"
+                             AcceptsReturn="True" Height="44" TextWrapping="Wrap" />
+                    <TextBlock Classes="hint" FontSize="10.5" TextWrapping="Wrap"
+                               Text="Added to the keychain as soon as you press ADD, so it stays even if you cancel this host. Renaming and deleting are on the keychain screen." />
+                    <StackPanel Orientation="Horizontal" Spacing="6">
+                      <Button Classes="accent" Content="ADD"
+                              Command="{Binding AddEditorCredentialCommand}" />
+                      <Button Classes="ghost" Content="CANCEL"
+                              Command="{Binding CancelEditorCredentialCommand}" />
+                    </StackPanel>
+                  </StackPanel>
+                </Border>
+
                 <!--
                   ◆ THE RELAY CARD, restyled to the mock's nested-card shape — radius 12, a checkbox with the
                   title beside it rather than under it — but NOT to the mock's copy. The sentence stays