Add SSH port forwarding (tunnels)

[timhartmann7]

Closes #104.

Local port forwarding, the ssh -L kind.

• Set forwards per host in the host editor (desktop) or the host form (TUI). LocalForward lines in ~/.ssh/config are picked up as they are.
• One tunnel per host carries all of its ports, and any number of hosts can run at once.
• Start and stop from the dashboard card, or with f in the TUI. A host can be set to start its tunnel when OmnySSH opens.
• A dropped connection is redialled with backoff while the local ports stay bound. A rejected login or a changed host key stops the tunnel instead of retrying.
• ProxyJump works the same as for every other connection.

Tested against OpenSSH in Docker: a service bound to the server's 127.0.0.1 (the Portainer case) and a Postgres behind the server are both reachable on localhost, also through a bastion, and tunnels recover after an sshd restart and a silent network drop.

Not included: remote (-R) and SOCKS (-D) forwarding.