Add shadcn sidebar/breadcrumb/collapsible/avatar/tooltip (base-nova)

Registry-installed Base UI flavor; existing ui components untouched.
Also drops the meaningless empty registries key from components.json.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/ui/collapsible.tsx

@@ -0,0 +1,19 @@
+import { Collapsible as CollapsiblePrimitive } from "@base-ui/react/collapsible"
+
+function Collapsible({ ...props }: CollapsiblePrimitive.Root.Props) {
+  return <CollapsiblePrimitive.Root data-slot="collapsible" {...props} />
+}
+
+function CollapsibleTrigger({ ...props }: CollapsiblePrimitive.Trigger.Props) {
+  return (
+    <CollapsiblePrimitive.Trigger data-slot="collapsible-trigger" {...props} />
+  )
+}
+
+function CollapsibleContent({ ...props }: CollapsiblePrimitive.Panel.Props) {
+  return (
+    <CollapsiblePrimitive.Panel data-slot="collapsible-content" {...props} />
+  )
+}
+
+export { Collapsible, CollapsibleTrigger, CollapsibleContent }