Agentic UI: Add preview sites to site overview

[shaunandrews]

Related issues

• None.

How AI was used in this PR

Codex helped extract the Preview Sites feature from the larger overview exploration into this focused stacked change. I reviewed the resulting scope and verified the behavior with automated tests and the local Agentic UI in both themes.

Proposed Changes

• Add a Preview Sites summary to the site overview so users can publish a shareable preview directly from the overview.
• Show previews belonging to the current site with their remaining lifetime, account usage, and clear expired states.
• Provide direct actions to open, copy, update, or republish preview links, while respecting signed-out and offline states.

Testing Instructions

1. Open a site's Overview tab while signed in.
2. Confirm the Preview sites section shows the empty state and publish action when the site has no previews.
3. Publish a preview and confirm its link, expiry, open, copy, and update actions.
4. Verify the signed-out explanation and offline-disabled publishing behavior.
5. Check the section in both light and dark appearance modes.

Automated verification:

• npx eslint --fix apps/ui/src/components/site-overview-view/preview-sites-section.tsx apps/ui/src/components/site-overview-view/index.tsx apps/ui/src/components/site-overview-view/index.test.tsx apps/ui/src/components/site-overview-view/cards.module.css
• npm test -- apps/ui/src/components/site-overview-view/index.test.tsx
• npm run typecheck
• npm run cli:build:ui

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?