feat: add download file method for files generated by plugin

lib/plugin-sandbox/host-api.ts

@@ -60,6 +60,7 @@ const PERM_PER_METHOD: Record<string, Permission | null> = {
   'ui.prompt': null,
   'ui.rerenderEmail': null,
   'ui.openExternalUrl': null,
+  'ui.downloadFile': 'ui:download-file'
 };
 
 function hasPermission(plugin: InstalledPlugin, perm: Permission): boolean {
@@ -400,6 +401,24 @@ async function saveFile(formerFileID:string, file: File): Promise<string> {
   return fileId;
 }
 
+// ─── Download files generated by the plugin. This is not user's files or attachments ──────────────────────────
+async function downloadFile(args: { content: string; filename: string; contentType?: string }): Promise<void> {
+    const { content, filename, contentType = 'application/json' } = args;
+
+    try {
+      const url = URL.createObjectURL(new Blob([content], { type: contentType }));
+      const a = document.createElement('a');
+      a.href = url;
+      a.download = filename;
+      document.body.appendChild(a); 
+      a.click();
+      document.body.removeChild(a);
+      URL.revokeObjectURL(url);
+    } catch (error) {
+     throw new Error(`Failed to download file: ${error}`);
+    }
+}
+
 // ─── admin config (same as before) ────────────────────────────
 
 async function adminGetAll(pluginId: string): Promise<Record<string, unknown>> {
@@ -547,6 +566,10 @@ export async function dispatchApiCall(
       window.open(parsed.toString(), '_blank', 'noopener,noreferrer');
       return undefined;
     }
+    case 'ui.downloadFile': {
+      const opts = args[0] as { content: string; filename: string; contentType?: string };
+      return downloadFile(opts);
+    }
 
     default:
       throw new Error(`Unhandled method "${method}"`);