修复折叠态跨月周选中和展开闪跳问题

1. 展开同步逻辑移至 CalendarPager（LaunchedEffect(Unit)），减少闪帧
2. 跨月周根据滑动方向选中日期：
   - 后退到跨月周：选较晚月份1号，留在当月
   - 前进到跨月周：选该周周一，留在上个月

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarMonthView.kt

@@ -9,7 +9,6 @@ import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.statusBarsPadding
 import androidx.compose.foundation.pager.rememberPagerState
 import androidx.compose.runtime.Composable
-import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableIntStateOf
@@ -61,22 +60,6 @@ fun CalendarMonthView(
 
     val pagerState = rememberPagerState(initialPage = START_PAGE, pageCount = { Int.MAX_VALUE })
 
-    // 展开时同步 CalendarPager 页面到 selectedDate 所在月份
-    LaunchedEffect(viewModel.isCollapsed) {
-        if (!viewModel.isCollapsed) {
-            @Suppress("DEPRECATION") // monthNumber 无替代 API
-            val targetPage = yearMonthToPage(
-                viewModel.selectedDate.year,
-                viewModel.selectedDate.month.number,
-                today.year,
-                today.month.number
-            )
-            if (targetPage != pagerState.currentPage) {
-                pagerState.scrollToPage(targetPage)
-            }
-        }
-    }
-
     val collapseProgress = viewModel.collapseProgress
     val headerHeightPx = monthHeaderHeightPx + weekdayHeaderHeightPx
     val rowPaddingPx = with(density) { ROW_PADDING_DP.dp.toPx() }.toInt()
@@ -179,9 +162,21 @@ fun CalendarMonthView(
                     today = today,
                     onDateClick = { date -> viewModel.selectDate(date) },
                     onWeekChanged = { weekMonday ->
-                        // 优先选中当周内的今天，否则选中该周周一
                         val weekSunday = weekMonday.plus(DatePeriod(days = 6))
-                        val date = if (today in weekMonday..weekSunday) today else weekMonday
+                        val date = when {
+                            today in weekMonday..weekSunday -> today
+                            weekMonday.month != weekSunday.month -> {
+                                if (weekMonday < viewModel.selectedDate) {
+                                    // 后退到跨月周（如从5月回到4月27-5月3）：选较晚月份1号，留在当月
+                                    @Suppress("DEPRECATION") // monthNumber 无替代 API
+                                    LocalDate(weekSunday.year, weekSunday.month.number, 1)
+                                } else {
+                                    // 前进到跨月周（如从4月前进到4月27-5月3）：选该周周一，留在上个月
+                                    weekMonday
+                                }
+                            }
+                            else -> weekMonday
+                        }
                         viewModel.selectDate(date)
                     },
                     modifier = pagerModifier

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarPager.kt

@@ -52,6 +52,17 @@ fun CalendarPager(
     val initialMonth = remember { today.month.number }
     val coroutineScope = rememberCoroutineScope()
 
+    // 展开后同步页面到 selectedDate 所在月份（修复折叠态切月后展开闪跳）
+    LaunchedEffect(Unit) {
+        @Suppress("DEPRECATION") // monthNumber 无替代 API
+        val targetPage = yearMonthToPage(
+            selectedDate.year, selectedDate.month.number, initialYear, initialMonth
+        )
+        if (targetPage != pagerState.currentPage) {
+            pagerState.scrollToPage(targetPage)
+        }
+    }
+
     // 跳过初始发射，保留首次渲染时的"今天"选中状态
     LaunchedEffect(pagerState) {
         snapshotFlow { pagerState.settledPage }.drop(1).collect { page ->