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.

tests/DodoSSH.Client.Terminal.Tests/FakeShellSession.cs

@@ -224,6 +224,17 @@ internal sealed class FakeConnection(SshConnectionRequest request, long bytesPer
         return Task.FromResult<ISshShellSession>(Shell);
     }
 
+    /// <summary>The forwards started on this connection, in order.</summary>
+    internal List<SshPortForwardSpec> Forwards { get; } = [];
+
+    /// <inheritdoc />
+    public Task<ISshPortForward> StartForwardAsync(SshPortForwardSpec spec, CancellationToken cancellationToken)
+    {
+        Forwards.Add(spec);
+
+        return Task.FromResult<ISshPortForward>(new FakePortForward(spec));
+    }
+
     /// <inheritdoc />
     public ValueTask DisposeAsync()
     {
@@ -361,3 +372,33 @@ internal sealed class DelegateProgress<T>(Action<T> report) : IProgress<T>
 {
     public void Report(T value) => report(value);
 }
+
+/// <summary>A forward that listens on nothing and says it is listening until disposed.</summary>
+internal sealed class FakePortForward(SshPortForwardSpec spec) : ISshPortForward
+{
+    /// <inheritdoc />
+    public event EventHandler<SshPortForwardErrorEventArgs>? ErrorOccurred
+    {
+        add { }
+        remove { }
+    }
+
+    /// <inheritdoc />
+    public SshPortForwardSpec Spec { get; } = spec;
+
+    /// <inheritdoc />
+    public int BoundPort => Spec.BindPort;
+
+    /// <inheritdoc />
+    public bool IsListening { get; private set; } = true;
+
+    /// <inheritdoc />
+    public string? LastError => null;
+
+    /// <inheritdoc />
+    public ValueTask DisposeAsync()
+    {
+        IsListening = false;
+        return ValueTask.CompletedTask;
+    }
+}