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.Ssh/SshNetConnectionFactory.cs

@@ -417,6 +417,14 @@ internal sealed class SshNetConnection(
         return Task.FromResult<ISshShellSession>(new SshNetShellSession(shell, client));
     }
 
+    /// <inheritdoc />
+    public Task<ISshPortForward> StartForwardAsync(SshPortForwardSpec spec, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(spec);
+
+        return SshNetPortForward.StartAsync(client, spec, cancellationToken);
+    }
+
     /// <inheritdoc />
     public async ValueTask DisposeAsync()
     {