Network Postcard Node.js web app

Build a small dependency-free Node.js web application named Network Postcard. Use node:http, a package.json with npm start, and listen on 0.0.0.0 using process.env.PORT (default 3000). Serve a polished accessible single-page dashboard and GET /api/network. That endpoint must make a real server-side HTTPS fetch to https://cdn.jsdelivr.net/npm/normalize.css@8.0.1/normalize.css with a 10-second timeout and return JSON containing ok, upstream status, and byte count; report errors honestly, never simulate success. The page must include the same external stylesheet directly from jsDelivr and display the server network result after clicking a clearly labeled Check connection button, with loading, success and error states. Include a local health endpoint. Keep everything small, use no database, no npm dependencies, and no build step. Add a concise README and run a local HTTP smoke test proving the health endpoint and network endpoint work. Deliver the working Node app for preview.

Validate and deliver the existing Network Postcard application from the seeded workspace. Preserve its design and dependency-free Node implementation. The previous GitHub publication omitted index.html and app.js although server.js serves them: ensure the complete application files are present. Make only necessary corrections, and update README.md with the exact start command and the endpoints verified. Use one concise implementation/verification phase. Start the existing server, verify /health, /, /app.js and the existing network-check endpoint; use the browser tool once to verify the visible page and Check connection interaction. Retain the machine-recorded probe evidence and deliver the complete working application. Do not add dependencies or redesign the site.

Extend the existing delivered Network Postcard with one small feature: Export latest result. Preserve the current dependency-free Node server, layout, /health and /api/network. After Check connection finishes, enable an Export JSON button that downloads the actual latest result with its timestamp, HTTP status and received byte count. Before any result exists, the export button must be disabled. Use a Blob download in the browser; do not add dependencies or server file writes. Update the README. Use one concise implementation and verification phase: start the app, check /health and /api/network, and use browser verification to click Check connection and confirm the visible result plus enabled export button. Preserve the machine-recorded probes and deliver the complete working files.

Fix one reproduced defect in the existing Network Postcard app.js: after a successful check, a subsequent fetch rejection or unreadable JSON leaves latestResult pointing to the old successful measurement, so Export JSON lies about the latest attempt. Make every completed attempt replace the previous result with its own timestamp, HTTP status (null when no response exists), received byte count and success/error. Preserve the existing UI, Blob download, dependency-free Node server and endpoint contracts. Add a dependency-free node:test regression suite that executes the real app.js with minimal DOM/fetch stubs and proves success then transport failure, success then invalid JSON, and successful recovery. Verify exported Blob contents, not only internal state. Add npm test and document it. First reproduce the old failure, then fix and run the tests. Use one bounded implementation/verification phase. Record the test command with record_probe; also start the server, verify /health and /api/network and browser-check the initial disabled export and successful check/export. In browser verification await checkConnection() or poll completion with a bounded timeout before asserting; waitMs only waits before clicks. Deliver all application and test files.

Verify and deliver the existing Network Postcard without adding features. Preserve all six delivered files and the three existing regression tests. Use one concise verification phase: run npm test and record the actual result with record_probe; start server.js, check /health and /api/network; browser-check that Export JSON starts disabled, await the real Check connection request, then capture the exported Blob and verify its timestamp, HTTP status and byte count match the displayed successful result. Use an async smoke that awaits window.__networkPostcard.checkConnection() before exporting, rather than fixed sleeps. Update README only with the verified commands and outcomes. Do not replace tests or rewrite application code unless a reproduced defect requires it. Deliver the complete app and test files.

Fix the README run command: server.js reads process.env.PORT, whereas LISTENING_ON_PORT is only its log message. Document the runnable example PORT=3000 npm start and http://localhost:3000/. Preserve all other application files and all three existing tests. Keep this as one small update-and-verification phase. Read server.js to confirm the input, update README, run npm test with record_probe and verify the documented startup configuration against the actual server. Then perform one browser verification of the existing app: initial Export JSON disabled, await the real checkConnection() request, capture the exported Blob, and compare timestamp, HTTP status and byte count against the displayed result. Return all checks in one structured smokeResult with aggregate ok; use deterministic equality booleans rather than raw changing timestamps. Do not add features or replace tests. Deliver the six complete files, and report only observed outcomes.