feat: sort tasks by due time

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

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { buildTaskRrule, defaultTaskDueAt, filterTasks, groupTaskTree, moveItemWithinScope, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
+import { buildTaskRrule, classifyTaskForToday, defaultTaskDueAt, filterTasks, groupTaskTree, isSameTaskSortTier, moveItemWithinScope, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
 
 type SearchTask = {
   id: string
@@ -33,6 +33,25 @@ describe('task utilities', () => {
     expect(groupTaskTree([parent])).toEqual([{ task: parent, subtasks: [child] }])
   })
 
+  it('classifies due edits for Today membership', () => {
+    const start = new Date('2026-09-10T00:00:00+08:00')
+    const end = new Date('2026-09-11T00:00:00+08:00')
+    expect(classifyTaskForToday({ completed: false, due_at: '2026-09-09T12:00:00+08:00' }, start, end)).toBe('overdue')
+    expect(classifyTaskForToday({ completed: false, due_at: '2026-09-10T12:00:00+08:00' }, start, end)).toBe('today')
+    expect(classifyTaskForToday({ completed: false, due_at: '2026-09-11T12:00:00+08:00' }, start, end)).toBe('outside')
+    expect(classifyTaskForToday({ completed: true, due_at: '2026-09-09T12:00:00+08:00' }, start, end)).toBe('outside')
+  })
+
+  it('only allows manual movement within the same completion and deadline tier', () => {
+    const noDueOpen = { completed: false, due_at: null }
+    expect(isSameTaskSortTier(noDueOpen, { completed: false, due_at: null })).toBe(true)
+    expect(isSameTaskSortTier({ completed: false, due_at: '2026-09-10T08:00:00Z' }, { completed: false, due_at: '2026-09-10T08:00:00Z' })).toBe(true)
+    expect(isSameTaskSortTier({ completed: false, due_at: '2026-09-10T08:00:00Z' }, { completed: false, due_at: '2026-09-10T16:00:00+08:00' })).toBe(true)
+    expect(isSameTaskSortTier(noDueOpen, { completed: false, due_at: '2026-09-10T08:00:00Z' })).toBe(false)
+    expect(isSameTaskSortTier(noDueOpen, { completed: true, due_at: null })).toBe(false)
+    expect(isSameTaskSortTier({ completed: false, due_at: '2026-09-10T08:00:00Z' }, { completed: false, due_at: '2026-09-11T08:00:00Z' })).toBe(false)
+  })
+
   it('moves an item before or after another item without changing other scopes', () => {
     const rows = [
       { id: 'a', parent_id: null },

frontend/src/lib/task-utils.ts

@@ -4,6 +4,7 @@ export type MinimalTask = {
   description?: string
   parent_id?: string | null
   completed?: boolean
+  due_at?: string | null
   list_name?: string
   subtasks?: MinimalTask[]
 }
@@ -92,6 +93,30 @@ export function groupTaskTree<T extends MinimalTask>(tasks: T[]) {
   }))
 }
 
+export function classifyTaskForToday(
+  task: Pick<MinimalTask, 'completed' | 'due_at'>,
+  start: Date,
+  end: Date,
+): 'overdue' | 'today' | 'outside' {
+  if (task.completed || !task.due_at) return 'outside'
+  const due = Date.parse(task.due_at)
+  if (!Number.isFinite(due)) return 'outside'
+  if (due < start.valueOf()) return 'overdue'
+  return due < end.valueOf() ? 'today' : 'outside'
+}
+
+export function isSameTaskSortTier(
+  source: Pick<MinimalTask, 'completed' | 'due_at'>,
+  target: Pick<MinimalTask, 'completed' | 'due_at'>,
+) {
+  if (Boolean(source.completed) !== Boolean(target.completed)) return false
+  if (!source.due_at && !target.due_at) return true
+  if (!source.due_at || !target.due_at) return false
+  const sourceTime = Date.parse(source.due_at)
+  const targetTime = Date.parse(target.due_at)
+  return Number.isFinite(sourceTime) && Number.isFinite(targetTime) && sourceTime === targetTime
+}
+
 export function moveItemWithinScope<T extends { id: string; parent_id?: string | null }>(items: T[], sourceId: string, targetId: string, placement: 'before' | 'after') {
   if (sourceId === targetId) return items
   const source = items.find((item) => item.id === sourceId)