Move Playground tabs into the sidebar as collapsible sub-items and make
the sidebar the sole navigation for playground interfaces.
- add collapsible UI primitive (bits-ui wrapper)
- add playground store with selected tab and menu open state (persistent)
- make Playground menu item collapsible; whole button toggles expand state
- move playground sub-items (Chat/Images/Speech/etc) under Playground
- remove in-page Tabs from Playground.svelte
- update sectionTitle breadcrumb to reflect active sub-item
- remove bg-sidebar panel background so items sit on page background
- remove persistent data-active background tint on menu items
fixes#123
Benson Wong