Seed threshold_configs["default"] from THRESHOLD_DEFAULTS at the start
of _parse_config() so the Settings page displays built-in defaults
regardless of whether the server config uses the multi-config format,
the legacy thresholds: format, or has no threshold config at all.
_parse_multi_config() overwrites the seed with the fully-merged
effective defaults when a threshold_configs section is present.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>