fix: Persist sidebar state using cookies for initial load

[bayashiko-r]

Hi @shadcn,

This pull request updates the SidebarProvider logic to persist the sidebar's open/collapsed state across sessions by utilizing browser cookies.

Key Changes:

• Initial State Read: The internal state (_open) now uses a lazy initializer function with React.useState to read the saved state from the SIDEBAR_COOKIE_NAME cookie on component mount.
• Default Fallback: If the cookie is not present (first-time load), the state correctly falls back to the defaultOpen prop.
• State Write: The setOpen callback is updated to correctly set the sidebar state in the cookie whenever the state changes.
• Code Clarity: Updated internal comments to accurately reflect the uncontrolled state initialization logic (reading from the cookie).

closes Issue #8176

[vercel]

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