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.Domain.Tests/HostFactory.cs

@@ -21,6 +21,24 @@ internal static class HostFactory
     /// <summary>A second tag id, for the tests about two people tagging one host.</summary>
     internal static Guid EuWest { get; } = Guid.Parse("0192f0c8-9999-7c3d-8e4f-5a6b7c8d9e09");
 
+    /// <summary>A forward to the database, as the host would carry it.</summary>
+    internal static PortForwardRule Postgres { get; } = new()
+    {
+        Id = Guid.Parse("0192f0c8-aaaa-7c3d-8e4f-5a6b7c8d9e0a"),
+        Kind = PortForwardKind.Local,
+        BindPort = 5432,
+        TargetHost = "db.internal",
+        TargetPort = 5432,
+    };
+
+    /// <summary>A SOCKS forward, for the tests about two forwards on one host.</summary>
+    internal static PortForwardRule Socks { get; } = new()
+    {
+        Id = Guid.Parse("0192f0c8-bbbb-7c3d-8e4f-5a6b7c8d9e0b"),
+        Kind = PortForwardKind.Dynamic,
+        BindPort = 1080,
+    };
+
     /// <remarks>
     /// <paramref name="port"/> defaults to an explicit 22 rather than to null, so a test that says nothing
     /// about ports gets the host shape that existed before inheritance did — which is what nearly every
@@ -40,7 +58,8 @@ internal static class HostFactory
         bool? asksForPassword = null,
         Guid? groupId = null,
         Guid[]? tags = null,
-        string[]? pinnedPaths = null) =>
+        string[]? pinnedPaths = null,
+        PortForwardRule[]? portForwards = null) =>
         new()
         {
             Label = label,
@@ -59,6 +78,7 @@ internal static class HostFactory
             GroupId = groupId,
             TagIds = tags is null ? TagSet.Empty : TagSet.Create(tags),
             PinnedPaths = pinnedPaths is null ? PinnedPathList.Empty : PinnedPathList.Create(pinnedPaths),
+            PortForwards = portForwards is null ? PortForwardList.Empty : PortForwardList.Create(portForwards),
         };
 
     /// <summary>