PAP-2659: Warn before sending issue reply with no assignee

If the issue reply composer would post without any assignee, the first
Send press now surfaces a warning toast instead of submitting. A second
press confirms the user wants to proceed unassigned, and changing the
assignee resets the confirmation.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/context/ToastContext.tsx

@@ -186,6 +186,10 @@ export function useToastActions() {
   return context;
 }
 
+export function useOptionalToastActions() {
+  return useContext(ToastActionsContext);
+}
+
 export function useToast() {
   const toasts = useToastState();
   const actions = useToastActions();