[bug] Sidebar: No collapsible visibility on Mobile.

OK

BAD

Steps to reproduce:
1. Use Sidebar.Provider with default collapsible behavior.

2. Render Sidebar with several Sidebar.Group collapsible blocks, each with Sidebar.GroupLabel + Sidebar.GroupContent + nav links (defaultOpen is fine).

3. Resize to width ≤ 767px.

4. Open the mobile sidebar.

Result: You won't be able to view the items inside collapsible groups.

Why is this?
`grid-template-rows` does not resolve to `1fr`, instead resolves to `0fr`. No space to display the items.

Doing the change to the class in the browser's developer tools fixes it.

Impact:
`emdash-cms/admin` uses this:
https://github.com/emdash-cms/emdash/issues/101

Suggested fix:
Treat the open mobile drawer like the expanded desktop sidebar: Either put the same group/sidebar + data-state="expanded" on its root, or stop tying collapsible group height to group-data-[state=expanded]/sidebar when you’re on the mobile sheet.

Do you think @ask-bonk could take this one up? 👀