feat(skill): add oRPC contract and hook for file download URL

Add frontend oRPC integration for the existing backend download URL
endpoint to enable file downloads from the asset tree.

web/types/app-asset.ts

@@ -70,6 +70,14 @@ export type AppAssetFileContentResponse = {
   content: string
 }
 
+/**
+ * File download URL response (GET /apps/{app_id}/assets/files/{node_id}/download-url)
+ */
+export type AppAssetFileDownloadUrlResponse = {
+  /** Presigned download URL */
+  download_url: string
+}
+
 /**
  * Delete node response (DELETE /apps/{app_id}/assets/nodes/{node_id})
  */