feat(portal-shell): add MSW mock layer with production bundle exclusion (P1-5)

MSW v2.7.0 fallback layer covering dashboard/users/exams/grades domains.
NEXT_PUBLIC_MSW=1 enables browser Service Worker + SSR route handler mock
responses without backend. Production build excludes all mock data via
Turbopack resolveAlias redirecting @/mocks to empty stub.

Acceptance: build bundle (client+server) verified clean of mock strings;
typecheck/lint/vitest (231 tests) all pass.

apps/portal-shell/next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/dev/types/routes.d.ts";
+import "./.next/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.