Docs: refresh screenshots for 1.12.0

[SandObserver]

Summary

• Replace the Introduction dashboard capture with a 1.12.0 capture of the same layout, shipped as public/img/dashboard-c1303e4c.jpg.
• Recapture the admin screenshots under public/img/admin/ from 1.12.0 in both colour schemes, with the same sections and seeded items as before.
• Give every refreshed image a content-hashed filename and update its references, because /img/* is served immutable for a year.
• Rebuild og.jpg (now og-<hash>.jpg) and .github/social-card.png around the new captures.

Testing
npm run build and npm run check both pass. The docs pages were not re-checked in a browser in both colour schemes and docs themes.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

[cloudflare-workers-and-pages]

Deploying stackyard-docs with    Cloudflare Pages

Latest commit:	6f0e660
Status:	✅  Deploy successful!
Preview URL:	https://53f01b28.stackyard-docs.pages.dev
Branch Preview URL:	https://docs-refresh-screenshots-1-1.stackyard-docs.pages.dev

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