feat: add reproducible local demo and recorded walkthrough

[biancarosa]

Summary

Netkit now has a reproducible local demo: run make demo and open http://localhost:13000. The launcher builds the real dashboard and proxy, starts a loopback store API, seeds four real HTTP exchanges, and verifies fresh capture records before reporting readiness.

• Demonstrates HTTP 200 catalog reads, HTTP 201 JSON POST capture, an 800 ms upstream delay, and an upstream HTTP 503.
• Adds make demo-verify and CI smoke coverage for fresh history records, request/response bodies, timing, statistics, and Prometheus metrics.
• Includes an 48-second recorded browser walkthrough, showing capture inspection, a new request sent through Compose, and the updated Overview.
• Simplifies the main README, adds full-resolution screenshots extracted from the video, and moves detailed dashboard/HTTPS setup into a linked guide.
• Documents prerequisites, ports, startup/shutdown, and how to record another walkthrough.

Validation

• Production dashboard build passed (including lint/type checks).
• go test ./... passed.
• Dashboard unit tests: 10 passed.
• Live demo verification passed repeatedly, requiring new request IDs each run.
• Launcher startup, SIGTERM cleanup, and release of all four ports passed.
• Browser confirmed the Compose response and five captured records in Overview.
• MP4: H.264, 1440×1000 at 30 fps, 48.4 seconds; full ffmpeg decode passed.
• Python compilation and git diff --check passed.

Scope

This is a native local development demo, not a hosted deployment. Netkit's existing listeners bind all interfaces; the guide calls out trusted-machine use. HTTPS interception and alert delivery are outside this walkthrough. Existing npm dependency audit findings remain unchanged.