Bind previews to IPv4 loopback and move tweb-debug to 9006

Vite's default host, `localhost`, binds only [::1] on macOS, so a preview did
not answer anything that dials the literal 127.0.0.1. start-preview.sh and the
plain-vite tweb-popups entry now pass `--host 127.0.0.1`: a preview answers
either form of the address and is still reachable from loopback only.

tweb-debug moves from 9011 to 9006, next to the other preview ports.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>