Use curated READMEs for app store detail pages

[Che-Zhu]

Summary

• add a readme URL field to generated app store app configs
• use curated README content written for each app instead of deriving the README from the upstream project repository
• preload that README markdown during static page generation and render it directly in the detail page
• remove the old README source metadata row from the detail page preview

Test Plan

• npm run lint
• node scripts/generate-apps-api.test.mjs
• npx tsx 'app/[lang]/products/app-store/[slug]/components/figma-gradient-heading.test.mts'
• production build/start manual verification of the app store list and /en/products/app-store/eaglercraft-server/

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fall back to GitHub source when configured README fails

This change makes app.readme authoritative: if the URL parses, getReadmeSource returns it immediately and loadReadmeMarkdown later converts any fetch failure into null, so the statically generated page permanently renders the screenshot fallback for that app build. In the app-store detail flow (page.tsx + loadReadmeMarkdown), a typo/404/transient outage on the configured README URL now suppresses the previously available GitHub/website candidate resolution path instead of falling back to it.

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