Header: the narrow-layout menu never shows primaryLinks (separate mobileMenuLinks slot that no app fills) #322

At narrow widths (below the header's 768 px breakpoint) the shell header drops every app's primary navigation links. The wide layout renders the org.openedx.frontend.slot.header.primaryLinks.v1 slot (desktop/PrimaryNavLinks.tsx). The narrow menu renders a different slot, org.openedx.frontend.slot.header.mobileMenuLinks.v1 (mobile/MobileNavLinks.tsx). No app fills that slot: searching the openedx org finds it only in frontend-base itself, where shell/header/app.tsx prepends the language menu. frontend-base's own dev header (shell/dev/devHeader/app.tsx) fills only primaryLinks.

Example (Tutor main, signed out, catalog home, 2026-10-16):

• 1280 px: the header shows Explore courses → /catalog/courses, from frontend-app-catalog's headerLinkExploreCourses widget, plus Sign Up and Login.
• 375 px: the toggle opens a menu containing only the language selector. "Explore courses" isn't anywhere in the header. The same applies to the catalog's signed-in links (Courses / Programs / Discover) and to learner-dashboard's primary links.
• Manually confirmed on a Pixel 10 against the master sandbox.

Impact: on a phone, the main navigation each app registers is unreachable from the header. For a signed-out visitor on the landing page, the full course catalog listing can only be reached by URL.

Expected: the narrow menu shows the same primary links as the wide layout. Options: render primaryLinks inside the mobile menu (or make mobileMenuLinks include it by default), or document that apps must register their links in both slots and update the first-party apps.

Environment: frontend-base main (header code as of 2026-09-22, #316), Tutor main (overhangio/openedx-mfe:22.1.0-main). First seen 2026-09-23 with an empty menu, before #316 added the language item. Found by the Open edX end-to-end test suite (BTR TC-00061).