feat(filesystem): add create-folder API for workspace picker

Adds a secure endpoint for creating a single subfolder in the current filesystem listing, and wires the non-native directory browser UI to create + enter the new folder.

packages/ui/src/lib/api-client.ts

@@ -8,6 +8,7 @@ import type {
   BinaryUpdateRequest,
   BinaryValidationResult,
   FileSystemEntry,
+  FileSystemCreateFolderResponse,
   FileSystemListResponse,
   InstanceData,
   ServerMeta,
@@ -224,6 +225,13 @@ export const serverApi = {
     const query = params.toString()
     return request<FileSystemListResponse>(query ? `/api/filesystem?${query}` : "/api/filesystem")
   },
+
+  createFileSystemFolder(parentPath: string | undefined, name: string): Promise<FileSystemCreateFolderResponse> {
+    return request<FileSystemCreateFolderResponse>("/api/filesystem/folders", {
+      method: "POST",
+      body: JSON.stringify({ parentPath, name }),
+    })
+  },
   readInstanceData(id: string): Promise<InstanceData> {
     return request<InstanceData>(`/api/storage/instances/${encodeURIComponent(id)}`)
   },