feat(ui): add collapsible component

Add Radix-based collapsible component for accordion-style UI sections.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/ui/src/components/ui/collapsible.tsx

@@ -0,0 +1,21 @@
+"use client"
+
+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 }