Terminal: ask what to connect to when Terminal is pressed with nothing open

The terminal surface with no session used to be an empty frame, with the tab row's "+" the only
thing on it that did anything. It now holds a prompt, in the order ssh asks: "connect to:" takes a
saved host's name, matched exactly, with the hosts that match so far listed under it for Tab or a
click, or user@host[:port] for a machine that is not saved. A password line follows only when one
is needed, worded as ssh words it. Escape steps back a line; recent connections are listed below.

It is drawn by Avalonia in the WebView's rectangle, which the occlusion rule allows because the two
are exclusive: the prompt shows only while there are no tabs, the WebView only for a tab with a
session.

Saved hosts connect through a new VaultViewModel.ConnectToHostAsync, which the drawer's own connect
now goes through too, so every refusal is still made once. Each attempt now carries whether it may
act on the drawer's remember-password tick, and the prompt's may not: otherwise a tick left on in
a closed drawer would save a password typed at a prompt that never offered to keep anything.

Covered by nine ShellFlowTests cases and four ConnectPromptTests; manual check 7.10b.

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

@@ -285,9 +285,8 @@
                 <!--
                   v5c-4: two rows rather than three, the same as the SFTP wrapper above and for the same
                   reason — the host header is gone and the terminal has its 60 pixels. The empty state it
-                  used to print ("no terminals open · press + or Ctrl+K…") went with it rather than moving:
-                  this Grid is only drawn on the terminal surface, and the surface with no tab open already
-                  answers for itself in the tab row's own "+" and in the connecting card below.
+                  used to print ("no terminals open · press + or Ctrl+K…") went with it, and the surface with
+                  no tab open is answered by the prompt below instead — see ConnectPrompt.axaml.
                 -->
                 <!--
                   Three rows since the disconnect strip arrived, where v5c-4 left two. The extra one is Auto
@@ -309,6 +308,16 @@
                     <views:ConnectingCard x:Name="ConnectingPane"
                                           IsVisible="{Binding IsConnectingShowing, FallbackValue=False}" />
 
+                    <!--
+                      The third: nothing open at all, which is what pressing Terminal on the rail with no
+                      session reaches. A prompt asking what to connect to, where an empty frame used to be.
+                      Exclusive with the WebView for the same reason the card above is — no tab, no session —
+                      and FallbackValue=False for the reason the WebView's own binding gives. See
+                      ConnectPrompt.axaml.
+                    -->
+                    <views:ConnectPrompt x:Name="PromptPane"
+                                         IsVisible="{Binding IsConnectPromptShowing, FallbackValue=False}" />
+
                     <!--
                       One WebView hosting every terminal. Not one per tab: each WebView2 is a separate
                       browser process tree, so twenty tabs would cost twenty of them.