fix: centralize ttdy host

[oliverxchen]

• ttyd binds to 127.0.0.1 but vhs.go was opening Chrome at http://localhost:. This does not fail on all machines, but may fail when Chrome chooses IPv6 first.

• This fix centralizes the ttdy host in ttyHost, uses it for both the ttyd --interface and the browser URL, and adds vhs_test.go to ensure they stay aligned.

• I have read CONTRIBUTING.md.

• Bug fix, not applicable. I have created a discussion that was approved by a maintainer (for new features).

How to check whether your machine is susceptible to the bug

Start ttyd the same way VHS does, bound to IPv4 loopback only:

ttyd --port=19760 --interface 127.0.0.1 --once --writable "$SHELL"

In another terminal, test IPv4 and IPv6 localhost separately:

curl -4 -I http://localhost:19760
curl -6 -I http://localhost:19760
curl -g -I 'http://[::1]:19760'

A machine is likely affected when IPv4 works but IPv6 fails:

curl -4 -I http://localhost:19760
# HTTP/1.1 200 OK

curl -6 -I http://localhost:19760
# curl: (7) Failed to connect to localhost port 19760

curl -g -I 'http://[::1]:19760'
# curl: (7) Failed to connect to ::1 port 19760

This can combine badly if localhost resolves to ::1 before 127.0.0.1:

dscacheutil -q host -a name localhost

If ::1 appears before 127.0.0.1, Chrome may try IPv6 first when VHS opens http://localhost:<port>, while ttyd is only listening on 127.0.0.1.