perf: WeekPager alpha() → graphicsLayer 避免滑动重组

与 CalendarPager 同样的问题：周视图滑动时 alpha 每帧变化触发重组。

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

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

@@ -13,7 +13,7 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Modifier
-import androidx.compose.ui.draw.alpha
+import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.unit.dp
 import kotlinx.coroutines.flow.drop
 import kotlinx.datetime.DatePeriod
@@ -87,7 +87,7 @@ fun WeekPager(
         val weekMonday = pageToWeekMonday(page, initialWeekMonday)
         Row(
             modifier = Modifier
-                .alpha(alpha)
+                .graphicsLayer { this.alpha = alpha }
                 .fillMaxWidth()
                 .padding(vertical = ROW_PADDING_DP.dp)
         ) {