docs(frontend): the task-body comment no longer points at a class that was deleted (#2962)

`.editor-body` was removed from editor-shared.css in c5faaf3 — nothing used
it once TaskEditorView replaced it — so "Replace .editor-body for task
editor" now names something a reader cannot find. Say what .task-body is and
record that its predecessor is gone.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/views/TaskEditorView.vue

@@ -803,7 +803,8 @@ useEditorGuards(dirty, save);
   max-width: 1600px;
 }
 
-/* Replace .editor-body for task editor */
+/* The task editor's own body row. It began as a replacement for the shared
+   .editor-body, which nothing used afterwards and has since been deleted. */
 .task-body {
   flex: 1;
   min-height: 0;