feat: register help button in the header

Calls frontend-base's helpButtonSlotOperation to surface a Help link
in the header for instructor-dashboard routes, gated on the
instructor-dashboard role. The URL comes from getAppConfig().SUPPORT_URL,
which the backend resolves via MFE_CONFIG / MFE_CONFIG_OVERRIDES or the
help-tokens legacy fallback.

Refs openedx/frontend-base#245

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: brian-smith-tcril