feat: add manifest-room route with discrepancy panel

[iamasx]

Summary

• add a dedicated manifest-room route with grouped manifests, package rollups, and a compact discrepancy panel
• add mock manifest, package, and discrepancy data plus a home-page entry for the route
• cover the new route in tests and mock next/link in Vitest setup so App Router pages render under Next 16

Closes #210