Handle unmatched mutation requests as 404s

[kentcdodds]

Summary

• Add action handlers to all root splat routes so scanner POST/PUT/PATCH/DELETE requests to unmatched paths return normal 404 responses instead of Remix missing-action errors.
• Add smoke coverage for POST requests to an unmatched scanner-style path.
• Refresh the existing exercise smoke-test home-page assertion from stale Hello World text to the rendered Epic Notes heading.

Sentry

• Addresses noisy getInternalRouterError events for Sentry project epicshop, issue EPICSHOP-F0 / 7447885629.
• Example paths: /connectors/resource/index.php, /connectors/system/phpthumb.php, /RSC/...txt, /_next, /__rsc, /.action.

Validation

• npm run typecheck
• npm run lint
• npm test in exercises/01.form-validation/01.problem.form-validation