Sidebar remembers each post type separately

Small fixes to how the folder sidebar remembers its state.

What's fixed

• Expanded folders and sidebar width are now remembered separately for each folder-enabled post type, so moving between the Media Library and Posts no longer overwrites the other's tree
• Folder search in the sidebar is now case-insensitive — typing "invoice" matches "Invoices"
• The sidebar only stores UI state (expanded folders, sidebar width) in the browser instead of caching the whole folder tree, keeping browser storage lean