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/App.axaml.cs

@@ -132,7 +132,8 @@ public sealed partial class DodoSshApp : Avalonia.Application
         var keepAlive = new SessionKeepAlive(
             workspace,
             activeTransfers: () => viewModel.Transfers.ActiveTransfers,
-            holdsFileSession: () => viewModel.Transfers.HasLiveFileSession);
+            holdsFileSession: () => viewModel.Transfers.HasLiveFileSession,
+            listeningForwards: () => viewModel.ListeningForwardCount);
 
         // The other end of the same wire: the workspace announces its own sessions ending, and the queue
         // announces transfers appearing and finishing, and a Files session connecting or disconnecting.
@@ -148,6 +149,11 @@ public sealed partial class DodoSshApp : Avalonia.Application
         // in the first place, which was precisely the promise this whole arrangement exists to keep.
         viewModel.TerminalSessionOpened += (_, _) => keepAlive.Refresh();
 
+        // A forward starting, stopping or failing changes the notification's wording and nothing else — it
+        // rides a shell's connection, so the shell is already what holds the service up. The user is still
+        // entitled to know a phone in their pocket is holding ports open.
+        viewModel.ForwardsChanged += (_, _) => keepAlive.Refresh();
+
         keepAlive.Refresh();
     }