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/package.json

@@ -38,6 +38,7 @@
     "graphql": "^16.8.0",
     "jose": "^5.9.6",
     "lucide-react": "^0.562.0",
+    "msw": "2.7.0",
     "next": "^16.0.10",
     "next-intl": "^4.13.2",
     "next-themes": "^0.4.6",
@@ -69,5 +70,10 @@
     "tsx": "^4.0.0",
     "typescript": "^5.6.0",
     "vitest": "^2.0.0"
+  },
+  "msw": {
+    "workerDirectory": [
+      "public"
+    ]
   }
-}
+}