Fix Dropdown scrollbar, arrow navigation, and scroll speed

[rzueger]

Summary

• Scrollbar click closing the menu: clicking the scrollbar of the options list caused the input to blur, hiding the menu. Fixed with preventDefault on OptionsContainer mousedown to keep focus on the input, plus explicit setInputFocused(false) on selection so options still close after picking a value.
• Arrow keys navigating past rendered options: keyboard navigation iterated through all filtered options, but only the first optionsRenderLimit are rendered. Pressing arrow down past the last rendered option moved focus to invisible items. Fixed by slicing the navigation array to match the render limit.
• Slow scroll-into-view: the scroll-into-view library defaults to 1000ms animation. Reduced to 50ms for responsive keyboard navigation with just enough motion to maintain spatial context.
• Weak arrow key tests: replaced element-existence assertions (which always pass) with behavioral tests that verify arrow + Enter produces the correct selection. Added tests for the render limit boundary and scrollbar interaction. Dropdown test coverage: 84% → 95% statements, 74% → 89% branches.

Test plan

• npm test — 1749 tests pass
• npm run typecheck — clean
• Manual testing: scrollbar interaction keeps menu open, arrow keys wrap within rendered options, scroll follows focus instantly
• Cypress E2E — Dropdown exercised by departure/arrival wizard specs