Prevent dropping onto project column 0

Fixes: go-gitea#29853
Fixes: go-gitea#29870

Author: silverwind

web_src/js/features/repo-projects.js

@@ -63,6 +63,11 @@ async function initRepoProjectSortable() {
     ghostClass: 'card-ghost',
     delayOnTouchOnly: true,
     delay: 500,
+    onMove: (e) => {
+      // prevent dropping onto column 0 because it currently can not be moved and doing
+      // so would raise a 500 "project board does not exist" error on backend.
+      if (e.related.getAttribute('data-id') === '0') return false;
+    },
     onSort: async () => {
       boardColumns = mainBoard.getElementsByClassName('project-column');
       for (let i = 0; i < boardColumns.length; i++) {