Add toasts to UI (#25449)

Fixes https://github.com/go-gitea/gitea/issues/24353

In some case like async success/error, it is useful to show toasts in UI.

web_src/js/features/comp/ComboMarkdownEditor.js

@@ -8,6 +8,7 @@ import {handleGlobalEnterQuickSubmit} from './QuickSubmit.js';
 import {renderPreviewPanelContent} from '../repo-editor.js';
 import {easyMDEToolbarActions} from './EasyMDEToolbarActions.js';
 import {initTextExpander} from './TextExpander.js';
+import {showErrorToast} from '../../modules/toast.js';
 
 let elementIdCounter = 0;
 
@@ -26,7 +27,7 @@ export function validateTextareaNonEmpty($textarea) {
       $form[0]?.reportValidity();
     } else {
       // The alert won't hurt users too much, because we are dropping the EasyMDE and the check only occurs in a few places.
-      alert('Require non-empty content');
+      showErrorToast('Require non-empty content');
     }
     return false;
   }