Docs: allowed addresses while no password is set

[SandObserver]

Summary

• Security: add "Addresses without a password", covering which addresses are answered while no password is set, why, and how the first page load sets the list.
• Settings reference: add the Allowed Addresses row under General, Security.
• Troubleshooting: add "Stackyard does not answer on this address" with the fix steps, and two rows in Messages you might see. The app links to this section by its anchor.

Testing
npm run build and npm run check pass. Text only; labels and messages copied from the app's ui/i18n/en.json, rules checked against ui/js/host-names.js and api/src/host-check.js.

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:	8b65d11
Status:	✅  Deploy successful!
Preview URL:	https://f0502cc5.stackyard-docs.pages.dev
Branch Preview URL:	https://docs-allowed-addresses.stackyard-docs.pages.dev

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