Next.js migration: calculator app

[MaxGhenis]

Part of #811

Goal

Migrate the calculator app from React Router nested routes to Next.js App Router.

Tasks

• Set up calculator route group (e.g. app/(calculator)/)
• Port MetadataGuard as a layout or middleware check
• Port CalcOrchestratorProvider context
• Migrate calculator pages:
  • /:countryId/reports (list)
  • /:countryId/simulations (list)
  • /:countryId/policies (list)
  • /:countryId/populations (list)
  • /:countryId/report-output/:reportId/:subpage?/:view? (dynamic)
• Flatten nested pathway routing into file-based routes:
  • PolicyPathwayWrapper sub-routes
  • SimulationPathwayWrapper sub-routes
  • ReportPathwayWrapper sub-routes
• Port calculator-specific hooks (42 hooks, many use React Router)
• Replace useNavigate/useParams/useSearchParams with Next.js equivalents
• Verify React Query normalized cache works with Next.js
• Test report builder flow end-to-end

Complexity notes

• The calculator has the most complex routing: nested sub-routes within pathway wrappers
• Heavy use of React Router hooks (useNavigate, useParams, useSearchParams)
• CalcOrchestratorContext manages multi-step wizard state
• This is the hardest phase — consider breaking into sub-issues if needed