Wrong redirect location when HTTPS_PORT is set

If `HTTPS_PORT` is different from 443, the redirect location is wrong - it should append the `config.HttpsPort` here if it's not 443: https://github.com/basecamp/thruster/blob/main/internal/server.go#L148-L153

Start at internal/server.go lines 148-153 and inspect how the HTTPS redirect location is assembled when HTTPS_PORT differs from 443. Verify the resulting location includes the configured port for non-default HTTPS ports while preserving the existing behavior for port 443.

Written by the indexing model from the issue text.