Improve 404 route handling and coverage

[cybercraftsolutionsllc]

Summary

• tracks NotFoundPage views with PostHog path/search metadata for client-side 404 visibility
• adds component coverage for the NotFound page tracking event and back-home action
• tightens nginx SPA fallback so direct React routes still serve index.html while missing static assets return nginx 404s
• adds nginx config coverage for SPA fallback and static asset 404 behavior

Fixes #1294

Validation

• REACT_APP_IS_TEST=true NODE_ENV=test node node_modules\jest\bin\jest.js --runTestsByPath src\pages\notFound\index.spec.tsx src\pages\notFound\nginxConfig.spec.ts --runInBand --no-coverage --silent --transformIgnorePatterns "./svg/"
• npx eslint src\pages\notFound\index.tsx src\pages\notFound\index.spec.tsx src\pages\notFound\nginxConfig.spec.ts --max-warnings 100
• git diff --check