fix: default new task due date to today

frontend/src/lib/task-utils.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { buildTaskRrule, filterTasks, groupTaskTree, moveItemWithinScope, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
+import { buildTaskRrule, defaultTaskDueAt, filterTasks, groupTaskTree, moveItemWithinScope, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
 
 type SearchTask = {
   id: string
@@ -63,6 +63,11 @@ describe('task utilities', () => {
     expect(filterTasks(searchable, '工作').map((task) => task.id)).toEqual(['4'])
   })
 
+  it('defaults new tasks to today at noon in local time', () => {
+    const due = defaultTaskDueAt(new Date(2026, 8, 8, 21, 30))
+    expect(due).toBe('2026-09-08T12:00')
+  })
+
   it('formats API dates in local wall-clock time', () => {
     const original = process.env.TZ
     process.env.TZ = 'Asia/Shanghai'