Add first-run demo screenshots and short walkthrough video

[boggspa]

Create public-facing media for the repository and release page.

Acceptance criteria:

• Role picker screenshot is current.
• Host setup screenshot hides pairing codes and local hostnames.
• Pairing/approval flow is captured without exposing private network details.
• One short walkthrough video demonstrates local-first hosting and viewer connection.
• Docs/Demo.md is updated with the final assets.