ui: consolidate playground nav into sidebar

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

ui-svelte/src/lib/components/ui/collapsible/index.ts

@@ -0,0 +1,13 @@
+import Root from "./collapsible.svelte";
+import Trigger from "./collapsible-trigger.svelte";
+import Content from "./collapsible-content.svelte";
+
+export {
+	Root,
+	Trigger,
+	Content,
+	//
+	Root as Collapsible,
+	Trigger as CollapsibleTrigger,
+	Content as CollapsibleContent,
+};