Add port forwarding: local, remote and dynamic, per terminal session

A host can now carry port forwards (-L, -R, -D), saved inside its
encrypted payload as RelayTicketRequest.PortForwardId already assumed,
so they sync and merge per forward like tags and pinned paths. The host
schema moves to version 8 only for hosts that carry forwards.

ISshConnection gains StartForwardAsync over SSH.NET's forwarded ports.
Every listener binds loopback and there is no bind-address field: a
forward is something any member of a shared keychain can edit. A remote
forward's target is resolved here, IPv4 first, rather than by SSH.NET's
constructor, which takes the first DNS answer and can send ::1.

Forwards start with every terminal opened to the host and stop with its
connection; a reconnect starts them again. The session sidebar lists
them with state, failure reason and an on/off switch, the status bar
counts the listening ones, and the host editor and detail pane gain a
PORT FORWARDS section. A forward that cannot start fails alone.

The phone does not start forwards yet: it has no surface showing what
is listening, and its More screen now says so. No byte counts, since
SSH.NET keeps none.

src/DodoSSH.Client.Shell/ViewModels/TerminalTabViewModel.cs

@@ -1,3 +1,6 @@
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+using System.Globalization;
 using CommunityToolkit.Mvvm.ComponentModel;
 using DodoSSH.Client.Ssh;
 
@@ -326,6 +329,73 @@ internal sealed partial class TerminalTabViewModel : ObservableObject
     /// <summary>How many steps there are, for the card's track.</summary>
     internal int StepCount => Steps.Count;
 
+    /// <summary>
+    /// The host's saved forwards, as they run over this tab's current session.
+    /// </summary>
+    /// <remarks>
+    /// Replaced whole each time a session opens here — see <see cref="ReplaceForwards"/> — and emptied of
+    /// meaning, though not of rows, when it ends: a row that says it stopped with the session is more use
+    /// than one that silently disappeared.
+    /// </remarks>
+    internal ObservableCollection<PortForwardViewModel> Forwards { get; } = [];
+
+    /// <summary>How many of <see cref="Forwards"/> are listening now.</summary>
+    internal int ListeningForwardCount => Forwards.Count(forward => forward.IsListening);
+
+    /// <summary>
+    /// The status bar's forward count, or null when nothing is listening and there is nothing to say.
+    /// </summary>
+    internal string? ForwardsSummary => ListeningForwardCount switch
+    {
+        0 => null,
+        1 => "⇄ 1 FORWARD",
+        var count => string.Create(CultureInfo.InvariantCulture, $"⇄ {count} FORWARDS"),
+    };
+
+    /// <summary>Puts a new session's forwards on this tab, ending whatever the last session left.</summary>
+    internal void ReplaceForwards(IEnumerable<PortForwardViewModel> forwards)
+    {
+        EndForwards();
+
+        foreach (var forward in Forwards)
+        {
+            forward.PropertyChanged -= OnForwardChanged;
+        }
+
+        Forwards.Clear();
+
+        foreach (var forward in forwards)
+        {
+            forward.PropertyChanged += OnForwardChanged;
+            Forwards.Add(forward);
+        }
+
+        RaiseForwardCount();
+    }
+
+    /// <summary>Tells every forward its session is over; SSH.NET has already stopped the listeners.</summary>
+    private void EndForwards()
+    {
+        foreach (var forward in Forwards)
+        {
+            forward.SessionEnded();
+        }
+    }
+
+    private void OnForwardChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        if (e.PropertyName is nameof(PortForwardViewModel.State))
+        {
+            RaiseForwardCount();
+        }
+    }
+
+    private void RaiseForwardCount()
+    {
+        OnPropertyChanged(nameof(ListeningForwardCount));
+        OnPropertyChanged(nameof(ForwardsSummary));
+    }
+
     /// <inheritdoc cref="TerminalTabState" />
     [ObservableProperty]
     private TerminalTabState state;
@@ -603,6 +673,8 @@ internal sealed partial class TerminalTabViewModel : ObservableObject
         EndDescription = description;
         IsLive = false;
 
+        EndForwards();
+
         OnPropertyChanged(nameof(EndedDetail));
         OnPropertyChanged(nameof(EndedSummary));
         OnPropertyChanged(nameof(EndDescription));
@@ -638,6 +710,9 @@ internal sealed partial class TerminalTabViewModel : ObservableObject
         EndDescription = null;
         IsLive = false;
 
+        // The old session is being closed for its pane's sake, and its forwards go with its connection.
+        EndForwards();
+
         // Cleared with the session they were facts about. Keeping them would put the previous connection's
         // cipher in the status bar beside a handshake that has not negotiated one yet.
         StartedAt = null;