refactor(settings): Use react-router instead of reach/router

[xlisachan]

This pull request

Package Changes

• Removed: @reach/router, @types/reach__router, @types/react-router from fxa-settings
• Added: react-router@7.18.0 to fxa-settings, fxa-admin-panel, fxa-react (peer)
• Removed: react-router-dom@^6.26.0 from fxa-admin-panel

Route Definitions (App/index.tsx, Settings/index.tsx)

• <Router basepath="/"> → <Routes> + <Route path="..." element={...} />
• <Redirect from to noThrow /> → <Route path element={<Navigate to replace />} />
• RouteComponentProps removed from all component signatures (~80+ files)
• added at app root (index.tsx)

Navigation

• useNavigate/useLocation/Link imports → react-router
• Standalone navigate() from reach/router → useNavigate() hook or window.location.assign()
• NavigateFn → NavigateFunction, NavigateOptions<{}> → NavigateOptions
• navigateWithQuery() (non-hook) → uses a registered navigate ref from App; falls back to window.location.assign/replace before React mounts

Infrastructure

• ReachRouterWindow → RouterWindow (deprecated alias kept)
• ScrollToTop: removed default prop, uses useLocation from react-router
• useWebRedirect: uses window.location.href instead of location.href

Test Infrastructure (mocks.tsx)

• createHistory/createMemorySource/LocationProvider → createMemoryRouter/RouterProvider/MemoryRouter
• renderWithRouter returns { ...renderResult, router } — tests use router.navigate() and
  router.state.location
• jest.spyOn(ReactRouterDom, 'useNavigate') → jest.mock('react-router', ...)
• IntersectionObserver mock added to setupTests.tsx
• Tests needing router context wrapped in

Issue that this pull request solves

Closes: PAY-3773

Checklist

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• If applicable, I have modified or added tests which pass locally.