Add trackpad pinch zoom for preview devices

[BboyGT]

Summary

• listens for trackpad pinch-style wheel events on preview webviews
• maps those gestures to the existing zoom in/out Redux actions
• adds a small delta threshold to avoid jumping across multiple zoom levels too quickly

Fixes #1049.

Testing

• Ran git diff --check
• Not run locally: project dependencies were not installed in this checkout

[BboyGT]

I pushed fixes for the TypeScript listener typing issues that showed up in CI. Happy to tune the gesture threshold if maintainers prefer a different sensitivity.

[CLAassistant]

All committers have signed the CLA.