Docs: add the interface icon set to the design system

[SandObserver]

Summary

• Add an Icons section to the design system page: the full set, rendered at build time from the app's ui/js/icon-set.js, a rules table, and the steps for adding a widget icon.
• Add IconSet.astro, which imports the app file through readRepoFile and falls back to a link when the app repo is absent.
• Rewrite the manifest glyph table to describe the new icons.
• Sync the bundled widget and admin scripts from the app, including the System widget's drawn network arrows.

Testing
npm run build and npm run check pass. Checked the Icons section in Chrome in the dark and light schemes.

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.

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