feat(frontend): handle /reports without an id

[Seme30]

Part of #789.

Problem

/reports with no id is a dead end on both hosts. Razor returns 500. Next.js renders "Report not found / Missing or invalid report id."

Reports are reachable from search and home, but there is no working reports index.

Current gaps

• Razor GET /reports → 500 error page.
• Next.js GET /reports → not-found empty state that assumes a detail route.
• No list of seeded reports (30 in demo seed) on either UI.

Frontend / Razor references

• frontend/app/reports/page.tsx
• web/Pages/Reports/Index.cshtml
• GET /api/reports

Definition of done

• Visiting /reports without an id does not 500 and does not look like a missing detail record.
• Either render a report list from /api/reports, or show a clear empty/index state that matches the chosen product behavior on both hosts.
• Typecheck, lint, and tests cover the missing-id path.