fix(slash): reset index when out of bounds

[sindras]

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Summary

Addressing issue reported in #1536

How did you test this change?

• Storybook

Before: hover index was out of menu
https://github.com/user-attachments/assets/a3e21563-5749-4444-8476-50dc5e4abaf8

After: hover index reset to zero when out of bounds
https://github.com/user-attachments/assets/805f312a-5a96-4416-b480-f40bf103fcd5

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Latest commit: f1b3e83

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Name	Status	Preview	Comments	Updated (UTC)
milkdown-storybook	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 13, 2024 8:10am

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