fix: 去掉 CalendarPager ↔ WeekPager 切换，修复折叠闪烁

CalendarPager 在 collapseProgress=1.0 时只渲染 anchor 行，
效果和 WeekPager 相同，但避免了切换导致的视觉跳跃。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/src/main/kotlin/plus/rua/project/ui/CalendarMonthView.kt

@@ -520,48 +520,20 @@ private fun CalendarPagerArea(
         modifier
     }
 
-    if (isCollapsed && collapseProgress >= 1f) {
-        WeekPager(
-            selectedDate = selectedDate,
-            today = today,
-            onDateClick = onDateClick,
-            onWeekChanged = { weekMonday ->
-                val weekSunday = weekMonday.plus(DatePeriod(days = 6))
-                val date = when {
-                    today in weekMonday..weekSunday -> today
-                    weekMonday.month != weekSunday.month -> {
-                        if (weekMonday < selectedDate) {
-                            @Suppress("DEPRECATION") // monthNumber 无替代 API
-                            LocalDate(weekSunday.year, weekSunday.month.number, 1)
-                        } else {
-                            weekMonday
-                        }
-                    }
-
-                    else -> weekMonday
-                }
-                onDateClick(date)
-            },
-            shiftKindAt = shiftKindAt,
-            showLegalHoliday = showLegalHoliday,
-            modifier = pagerModifier
-        )
-    } else {
-        CalendarPager(
-            selectedDate = selectedDate,
-            today = today,
-            onDateClick = onDateClick,
-            onMonthChanged = onMonthChanged,
-            collapseProgress = collapseProgress,
-            rowHeightPx = rowHeightPx,
-            effectiveWeeks = effectiveWeeks,
-            shiftKindAt = shiftKindAt,
-            showLegalHoliday = showLegalHoliday,
-            onRowHeightMeasured = onRowHeightMeasured,
-            pagerState = pagerState,
-            modifier = pagerModifier
-        )
-    }
+    CalendarPager(
+        selectedDate = selectedDate,
+        today = today,
+        onDateClick = onDateClick,
+        onMonthChanged = onMonthChanged,
+        collapseProgress = collapseProgress,
+        rowHeightPx = rowHeightPx,
+        effectiveWeeks = effectiveWeeks,
+        shiftKindAt = shiftKindAt,
+        showLegalHoliday = showLegalHoliday,
+        onRowHeightMeasured = onRowHeightMeasured,
+        pagerState = pagerState,
+        modifier = pagerModifier
+    )
 }
 
 @Composable

core/src/main/kotlin/plus/rua/project/ui/WeekPager.kt

@@ -1,8 +1,10 @@
 package plus.rua.project.ui
 
+import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.foundation.pager.HorizontalPager
 import androidx.compose.foundation.pager.PagerDefaults
 import androidx.compose.foundation.pager.rememberPagerState
@@ -89,6 +91,7 @@ fun WeekPager(
             modifier = Modifier
                 .alpha(alpha)
                 .fillMaxWidth()
+                .background(MaterialTheme.colorScheme.surface)
                 .padding(vertical = ROW_PADDING_DP.dp)
         ) {
             (0 until 7).forEach { dayOffset ->