feat: add ALLOW_PRIVATE_PROXY_HOSTS config for local proxy testing

Add opt-in flag to allow private/internal hosts (192.168.x.x, 10.x.x.x, localhost) in dashboard proxy tests. Default remains public-only for security. When enabled, testProxy() uses validateHostFormat() instead of assertPublicUrlHost() — still validates DNS/IP format but skips SSRF guards.

New validateHostFormat() helper in net-safety.js mirrors resolvePublicAddresses() structure without private-range rejection. Config wired through .env.example, setup.sh, and config