Docs: sync with Stackyard 1.15.0

[SandObserver]

Summary

• Describe the Solid color swatches, the hue, saturation and brightness sliders and the Color Code field in the Settings reference.
• Add the HTTP 404 and generic status messages and the Unsplash wallpaper message to the Troubleshooting message table.
• List setCardAppearance(dark) in the widget toolbox reference.
• Refresh the embedded app code and widget copies from the 1.15.0 tree.

Testing
npm run build and npm run check pass.

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:	ef78a3c
Status:	✅  Deploy successful!
Preview URL:	https://29f9162b.stackyard-docs.pages.dev
Branch Preview URL:	https://docs-sync-1-15-0.stackyard-docs.pages.dev

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