fix: 给日历根容器和日期标签添加背景色，修复主题切换透底

- CalendarMonthView 根 Box 添加 background(MaterialTheme.colorScheme.background)
- DayCell 班次标签和农历标签添加同色背景

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

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

@@ -203,6 +203,7 @@ fun CalendarMonthView(
     Box(
         modifier = modifier
             .fillMaxSize()
+            .background(MaterialTheme.colorScheme.background)
             .statusBarsPadding()
             .onSizeChanged { size ->
                 screenWidthPx = size.width

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

@@ -5,6 +5,7 @@ import androidx.compose.animation.core.FastOutSlowInEasing
 import androidx.compose.animation.core.animateFloat
 import androidx.compose.animation.core.tween
 import androidx.compose.animation.core.updateTransition
+import androidx.compose.foundation.background
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.interaction.MutableInteractionSource
 import androidx.compose.foundation.layout.Box
@@ -291,6 +292,7 @@ fun DayCell(
                 modifier = Modifier
                     .align(shiftAlignment)
                     .zIndex(1f)
+                    .background(MaterialTheme.colorScheme.background)
                     .then(shiftPadding)
                     .padding(horizontal = 2.dp)
             )
@@ -305,6 +307,7 @@ fun DayCell(
                 modifier = Modifier
                     .align(Alignment.TopEnd)
                     .zIndex(1f)
+                    .background(MaterialTheme.colorScheme.background)
                     .padding(top = 1.dp, end = 2.dp)
                     .padding(horizontal = 2.dp)
             )