[grade=A] P2-6: refactor config-schema.js validateConfig (complexity 44→8 sub-validators)
Extracted 8 field-level validators from the monolithic validateConfig function:
validateTld, validateDns, validateDashboardHost, validateTimezone,
validateTheme, validateRoutingMode, validateDomain, validateKnownKeys.
ESLint complexity dropped from 44 (Error) to <10 per function.
Removed unused VALID_TIMEZONES_SAMPLE constant.
Extracted VALID_THEMES, VALID_ROUTING_MODES, VALID_DNS_PROVIDERS, KNOWN_KEYS
as module-level constants.
Behavior-preserving: same validation rules, same error/warning messages,
same return shape. 1539/1539 tests pass. ESLint: 0 problems (was 2).