Start port forwards on the phone, listed in a sheet off its terminal

The Android head skipped a host's forwards because it had nowhere to show
what was listening. Its terminal bar now draws a ⇄ for a tab that has
forwards, carrying the listening count in green and turning the danger
colour when one failed, and it raises a sheet over the same rows the
desktop sidebar lists: notation, route, state, failure reason and a tap
to switch each off or on. MainWindowViewModel.StartForwards no longer
returns early on Android.

The sheet is shell state like the connect sheet: it collapses the
renderer while up, is lowered by leaving the terminal, and the back
gesture closes it first.

Forwards ride the shell's keep-alive rather than holding the foreground
service themselves, since a forward cannot outlive its shell. They only
change the notification, which now counts ports forwarded beside shells
connected, fed by a new ForwardsChanged event and ListeningForwardCount.

The phone still has no editor for forwards; the More screen card, README
and design-import-gaps now say they run here and are edited on the
desktop.

src/DodoSSH.Client.Android/Views/TerminalScreen.axaml.cs

@@ -210,10 +210,14 @@ internal sealed partial class TerminalScreen : UserControl
     private void AdoptRenderer() => RendererFocus.Suppress(input.Acquire);
 
     /// <summary>
-    /// Puts the keyboard away on the way to the connect menu.
+    /// Puts the keyboard away on the way to the connect menu or the forwards sheet.
     /// </summary>
     /// <remarks>
     /// <para>
+    /// One handler for both buttons in the terminal's bar that raise a sheet, since what follows is true of
+    /// either: each is a panel at the bottom of the screen, which is where the keyboard is.
+    /// </para>
+    /// <para>
     /// A handler beside the button's command rather than something the command does, because the command is
     /// on the shared view model and this is a platform call: the desktop head has no software keyboard to
     /// put away, and a shell view model that knew about Android's input-method manager would be the first
@@ -222,7 +226,7 @@ internal sealed partial class TerminalScreen : UserControl
     /// <para>
     /// <b>The keyboard is up in the ordinary case.</b> This button sits above a terminal somebody is typing
     /// into, so the sheet it raises would otherwise arrive underneath a keyboard covering half the display —
-    /// and the three rows on it are the bottom half. Worse, the keyboard's inset shortens everything the
+    /// and the rows on it are the bottom half. Worse, the keyboard's inset shortens everything the
     /// phone draws (see <c>PhoneShell.ApplyKeyboardInset</c>), so the sheet would be laid out into the
     /// remaining strip rather than merely hidden behind it.
     /// </para>
@@ -231,7 +235,7 @@ internal sealed partial class TerminalScreen : UserControl
     /// what the other writes: this one talks to the platform and the command sets a flag, so they commute.
     /// </para>
     /// </remarks>
-    private void OnConnectMenuRequested(object? sender, RoutedEventArgs e) => SoftKeyboard.Hide();
+    private void OnSheetRequested(object? sender, RoutedEventArgs e) => SoftKeyboard.Hide();
 
     /// <summary>
     /// Takes the browser gestures back off a surface that is not a web page.