build: Default the dev MFE config API URL to local.openedx.io (#1981)

Fetching MFE config from local.openedx.io:8000 instead of localhost:8000
keeps the LMS, Studio, and MFEs under one *.local.openedx.io domain, which
avoids localhost-specific CORS/cookie carve-outs and better matches a
production setup.

Works with Tutor dev (whose default LMS host is local.openedx.io) and with
the upcoming openedx-platform bare-metal development.py config.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>