fix: replace items prop with ListItem children in homepage Select and ComboBox

[Bhavysinghal]

What

Fixes #11

The Select and ComboBox components on the homepage (index.mdx) were using an items prop that doesn't exist on either component. This caused both dropdowns to render with no options and not respond to clicks.

Why

The Select and ComboBox components from @umami/react-zen expect <ListItem> elements as children — not an items array prop. Passing an unsupported prop silently did nothing, leaving the dropdowns empty.

Fix

Replaced the items prop with proper <ListItem> children on both components in apps/docs/content/index.mdx.

Before

After

How to test

1. Run pnpm --filter react-zen-docs dev
2. Go to http://localhost:9001
3. Click the Country select → dropdown now shows: United States, United Kingdom, Canada, Australia
4. Click the Framework combobox → dropdown now shows: React, Vue, Angular, Svelte

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