Add real screenshots/thumbnails for demo apps (homepage + /examples)

[justin808]

Follow-up to #112, which added the demo/starter showcase on the homepage ("See it running") and the /examples page.

Each demo currently renders a branded placeholder tile instead of a real graphic. This issue tracks capturing and wiring real screenshots (or thumbnails) for each app.

What to do

For each demo below, capture a screenshot/thumbnail of the running app and add it as static/img/demos/<id>.png, then set image: '/img/demos/<id>.png' on the matching entry in prototypes/docusaurus/src/constants/demos.ts. No component changes are needed — DemoCard swaps the placeholder for the image automatically.

Recommended: a clean 16:9 hero shot of the app's main screen.

Demo	id	Source	Live
Marketplace	marketplace	react-on-rails-demo-marketplace-rsc	https://rsc.reactonrails.com
TanStack Starter	tanstack-starter	react-on-rails-starter-tanstack	https://start.reactonrails.com
Legacy tutorial	legacy-tutorial	react-webpack-rails-tutorial	https://www.reactrails.com
Hacker News	hacker-news	react-on-rails-demo-hacker-news-rsc	deploy pending
Gumroad	gumroad	react-on-rails-demo-gumroad-rsc	deploy pending

Notes

• The three live demos (Marketplace, TanStack, Legacy) can be screenshotted now.
• Hacker News and Gumroad need their deployments live first (they also still show "Demo coming soon" until demoUrl is set in the same data file).

[justin808]

Thumbnail audit update from the #129 flagship demo lane:

• PR Add the OSS flagship demo to examples #130 adds the remaining flagship OSS demo card plus /img/demos/flagship.webp: Add the OSS flagship demo to examples #130
• The branch audit found image assets wired for all current catalog entries: flagship, marketplace, tanstack-starter, legacy-tutorial, hacker-news, octochangelog, and gumroad.
• The new flagship asset is a real 1280x720 WebP screenshot from https://flagship.reactonrails.com.
• Local validation for the branch passed with npm run build; Docusaurus still reports the repo's existing broken-link/anchor warnings, but exits 0.
• Repo workflow state for this batch was UNKNOWN because .agents/skills/pr-batch/SKILL.md and .agents/workflows/pr-processing.md are absent locally and on origin/main; installed $pr-batch fallback concepts were used.
• Coordination: bounded agent-coord status timed out for the worker lane, so live backend state was UNKNOWN.

I am leaving this issue open until #130 lands, because main does not include the flagship thumbnail until that PR merges.