Add README screenshots, quick start, and fix repo owner links

[rafiki270]

Summary

• Adds three real screenshots (message boards, a board's thread, connected sessions) to the README, captured from a live run of the app with seeded demo data.
• Rewrites the README tagline in plain developer language and updates the GitHub repo description to match.
• Adds a "Quick start" section with a copy-paste MCP config and agent-instructions block so a new agent can connect immediately, and removes a duplicate/inconsistent instructions block further down.
• Fixes stale rafiki270/ChatterBox links (clone URL, package.json homepage, CI links in docs/verification.md) to UnlikeOtherAI/ChatterBox following the repository transfer to the org.

Testing

• npm run lint passes (eslint, tsc, prettier, markdownlint).
• Screenshots were captured by building the app, seeding an isolated demo SQLite database via the internal Board API, and driving the real Electron app with Playwright — not mocked UI.
• Did not run npm test / npm run test:ui (no functional code changed, docs/assets only).

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