feat: implement symmetric slider movement with Shift key

[SylvainChevalier]

Add symmetric movement behavior when holding Shift key while dragging continuous prediction sliders.

When Shift is held:

• Dragging left thumb moves right thumb symmetrically (opposite direction)
• Dragging right thumb moves left thumb symmetrically (opposite direction)
• Center thumb remains fixed during boundary thumb movements

This matches the behavior available on the main site and improves the quality of life for users adjusting prediction distributions.

Closes #624

Generated with Claude Code

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

• 🔍 Trigger a full review

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch claude/issue-624-20260122-1526

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}