Add screenshots and a demo GIF to the README

[sanmaxdev]

Background

The README explains the platform well but has no visuals. Screenshots of the dashboard, account view, and backtesting, plus a short demo GIF, would help people quickly understand the project and decide to try it.

Tasks

• Run the app locally (docker compose up -d) and capture a few clean screenshots
• Add an optional short demo GIF of the dashboard
• Place assets under docs/images/ and embed them in README.md

Acceptance criteria

• Images render correctly on GitHub
• No personal data or real API keys are visible in the captures

See CONTRIBUTING.md.