Docs: refresh the screenshots and social card

[SandObserver]

Summary

• Recapture docs/screenshot.png and the four Settings screenshots used by the README and the Unraid template from the current UI.
• Replace docs/social-card.png with a card built from the landing page.
• Recapture the install screenshots in ui/screenshots/.

Testing
All green, including the Docker build. Images only, no new tests.

Changelog

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.