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.

docs/android-port.md

@@ -631,7 +631,8 @@ go at 360dp:
    for. `ShellScreen` gained `More` and `Buckets`. The palette went blue and the two heads moved together,
    because it is shared. What v2 asked for and did not get is in
    [`docs/design-import-gaps.md`](design-import-gaps.md) — the short version being that port forwarding is
-   named as absent rather than drawn, because nothing in the SSH layer forwards anything.
+   named rather than drawn: it has since been built for the desktop, and this head does not start forwards
+   because it has no surface yet that shows what is listening.
 
    **The bar has since gone from four entries to three.** The keychain joined the hub, which is now called
    SETTINGS rather than MORE and wears a gear: a bottom bar is for the places a session moves between, and