feat: refine task and countdown interactions

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

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { applyMarkdownFormat, buildTaskRecurrencePayload, buildTaskRrule, classifyTaskForToday, defaultTaskDueAt, filterTasks, groupTaskTree, isSameTaskSortTier, mergeReorderedSubset, moveItemWithinScope, parseTaskRecurrence, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
+import { applyMarkdownFormat, buildTaskDueDraft, buildTaskRecurrencePayload, buildTaskRrule, classifyTaskForToday, defaultTaskDueAt, filterTasks, groupTaskTree, isSameTaskSortTier, mergeReorderedSubset, moveItemWithinScope, parseTaskDueDraft, parseTaskRecurrence, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
 
 type SearchTask = {
   id: string
@@ -136,4 +136,23 @@ describe('task utilities', () => {
     process.env.TZ = original
     expect(toDateTimeLocal(null)).toBe('')
   })
+
+  it('parses date-only and timed task deadlines without inventing a time', () => {
+    const original = process.env.TZ
+    process.env.TZ = 'Asia/Shanghai'
+    expect(parseTaskDueDraft('2026-09-05T15:59:00Z', false)).toEqual({ date: '2026-09-05', hasTime: false, time: '12:00' })
+    expect(parseTaskDueDraft('2026-09-05T12:30:00Z', true)).toEqual({ date: '2026-09-05', hasTime: true, time: '20:30' })
+    expect(parseTaskDueDraft(null, true)).toEqual({ date: '', hasTime: false, time: '12:00' })
+    process.env.TZ = original
+  })
+
+  it('builds date-only, timed, toggled and cleared task deadlines in local time', () => {
+    const original = process.env.TZ
+    process.env.TZ = 'Asia/Shanghai'
+    expect(buildTaskDueDraft({ date: '2026-09-05', hasTime: false, time: '08:15' })).toEqual({ due_at: '2026-09-05T15:59:00.000Z', due_has_time: false })
+    expect(buildTaskDueDraft({ date: '2026-09-05', hasTime: true, time: '08:15' })).toEqual({ due_at: '2026-09-05T00:15:00.000Z', due_has_time: true })
+    expect(buildTaskDueDraft({ date: '2026-09-05', hasTime: true, time: '' })).toEqual({ due_at: '2026-09-05T15:59:00.000Z', due_has_time: false })
+    expect(buildTaskDueDraft({ date: '', hasTime: true, time: '08:15' })).toEqual({ due_at: null, due_has_time: false })
+    process.env.TZ = original
+  })
 })