fix: keep today habits visible while refreshing

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

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { calendarModeLabel, clampFabPosition, countdownDayText, countdownKindLabel, dateKey, defaultView, habitButtonValue, habitWeek, isFabDrag, isHabitComplete, isHabitScheduledToday, isTaskView, nextHabitSwipeValue, numericHabitInputValue, previousHabitSwipeValue, quickTaskFields, shouldToggleRowSwipe } from './mvp-utils'
+import { calendarModeLabel, clampFabPosition, countdownDayText, countdownKindLabel, dateKey, defaultView, habitButtonValue, habitWeek, isFabDrag, isHabitComplete, isHabitScheduledToday, isTaskView, nextHabitSwipeValue, nextTotalAfterLocalTaskAdd, numericHabitInputValue, previousHabitSwipeValue, quickTaskFields, readHabitGridCache, shouldToggleRowSwipe, writeHabitGridCache } from './mvp-utils'
 
 describe('MVP view utilities', () => {
   it('formats a local date as YYYY-MM-DD', () => {
@@ -70,6 +70,13 @@ describe('MVP view utilities', () => {
     expect(habitButtonValue('boolean', 1, 1)).toBe(0)
   })
 
+  it('reuses the current week habit grid while refreshing in the background', () => {
+    const habits = [{ id: 'habit-1', name: '喝水' }]
+    writeHabitGridCache('2026-09-07', habits)
+    expect(readHabitGridCache('2026-09-07')).toEqual(habits)
+    expect(readHabitGridCache('2026-09-14')).toBeNull()
+  })
+
   it('renders countdown labels from date-only day values', () => {
     expect(countdownDayText(8)).toBe('还有 8 天')
     expect(countdownDayText(0)).toBe('就是今天')

frontend/src/lib/mvp-utils.ts

@@ -114,6 +114,16 @@ export function isFabDrag(deltaX: number, deltaY: number, threshold = 8) {
   return Math.hypot(deltaX, deltaY) >= threshold
 }
 
+let habitGridCache: { week: string; habits: unknown[] } | null = null
+
+export function readHabitGridCache<T>(week: string): T[] | null {
+  return habitGridCache?.week === week ? habitGridCache.habits as T[] : null
+}
+
+export function writeHabitGridCache<T>(week: string, habits: T[]) {
+  habitGridCache = { week, habits }
+}
+
 export function formatApiErrorDetail(detail: unknown): string {
   if (typeof detail === 'string') return detail
   if (Array.isArray(detail)) {