test(gui): replace fireEvent with userEvent in all slider and interactive component tests (t296)

[bwyard]

Summary

Several GUI component tests use @testing-library/react's fireEvent to simulate user interactions on sliders, buttons, and toggles. fireEvent fires a single DOM event and skips the browser's natural event sequence. userEvent simulates real user behaviour — pointerdown, pointermove, pointerup, focus, etc. — which catches more bugs and matches how Score Studio is actually used.

What to change

In packages/gui/tests/ and any *.test.tsx files:

// Before
fireEvent.change(slider, { target: { value: '0.8' } })

// After
import userEvent from '@testing-library/user-event'
const user = userEvent.setup()
await user.pointer([
  { target: slider, keys: '[MouseLeft>]', coords: { x: 100 } },
  { coords: { x: 200 } },
  { keys: '[/MouseLeft]' },
])

For button/toggle interactions:

// Before
fireEvent.click(muteButton)

// After
await user.click(muteButton)

Scope

Run this to find all fireEvent usages in GUI tests:

grep -rn "fireEvent" packages/gui/tests/

Replace each one with the appropriate userEvent equivalent. Each component can be a separate commit.

Notes

• @testing-library/user-event is already installed — check packages/gui/package.json
• Tests that use fireEvent.change on <input type="range"> (sliders) are the highest priority — slider drag behaviour is the most likely place real bugs hide
• Must not reduce test coverage — all existing assertions should still pass

Good first issue — mechanical change with a clear target list, improves test accuracy, no new features.