docs: refresh README frontend screenshots

[arthurpanhku]

Summary

• replace the stale README dashboard screenshot with the refactored frontend
• regenerate the Product Tour GIF from four real browser-tested states
• cover the overview, Cmd/Ctrl+K command search, new assessment dialog, and Agent Gateway
• preserve the existing README image paths so all consumers update automatically
• reduce the demo GIF from 326 KB to 306 KB while moving to a 16:9 viewport

Validation

• ran the FastAPI backend and Vite frontend together
• verified API health and the grouped navigation shell in a real browser
• exercised command search with the mcp query and navigated to Agent Gateway
• opened and inspected the New assessment dialog
• browser console: 0 warnings and 0 errors
• npm run check --prefix frontend: 8 tests passed
• npm run build --prefix frontend: passed
• npm audit --omit=dev --prefix frontend: 0 vulnerabilities
• GIF metadata: 4 frames, 960x540, infinite loop
• git diff --check: passed