fix(tasks): keep generate confirmation dialog open during in-flight request

AlertDialogAction closes the dialog synchronously on click before any
awaited handler logic runs, so the disabled/'Generating...' state on the
Confirm button was unreachable. Call event.preventDefault() in
handleConfirmGenerate to suppress Radix's auto-close; the dialog now
closes explicitly via setPreview(null) only on success, and stays open
(with the toast) on error so the user can retry or cancel.

ondeck/src/app/(dashboard)/tasks/assign/page-client.tsx

@@ -194,7 +194,8 @@ export function BulkAssignClient({ users, clients, designations }: BulkAssignCli
     }
   }
 
-  const handleConfirmGenerate = async () => {
+  const handleConfirmGenerate = async (event: React.MouseEvent) => {
+    event.preventDefault()
     setGenerating(true)
     try {
       const res = await fetch('/api/tasks/generate-and-assign', {