test: add Binance browser automation foundation

[jackhai9]

Summary

• add a layered UI automation design for the Binance orderbook userscript
• add a deterministic Binance Futures fixture that executes the generated userscript in Chromium
• cover cancel-current-symbol invariants, dialog outcomes, state restoration, single-flight behavior, layout stability, and interaction performance
• add a GitHub Actions browser-test gate with retained Playwright failure artifacts

Validation

• npm test (292/292)
• npm run test:ui -- --repeat-each=3 (18/18)
• npm run build:binance-orderbook-trade
• npm run check:binance-orderbook-trade
• git diff --check
• npm audit --omit=dev (0 vulnerabilities)

Scope

Test infrastructure only. No userscript source, generated artifact, or version change.