Add MonthHeader, WeekdayHeader, and DayCell UI components

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -0,0 +1,52 @@
+package plus.rua.project.ui
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.aspectRatio
+import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.text.style.TextAlign
+import kotlinx.datetime.LocalDate
+
+@Composable
+fun DayCell(
+    date: LocalDate,
+    isCurrentMonth: Boolean,
+    isSelected: Boolean,
+    isToday: Boolean,
+    onClick: () -> Unit,
+    modifier: Modifier = Modifier
+) {
+    val contentColor = when {
+        isSelected -> MaterialTheme.colorScheme.onPrimary
+        !isCurrentMonth -> MaterialTheme.colorScheme.onSurface.copy(alpha = 0.38f)
+        else -> MaterialTheme.colorScheme.onSurface
+    }
+
+    val backgroundColor = when {
+        isSelected -> MaterialTheme.colorScheme.primary
+        else -> MaterialTheme.colorScheme.surface
+    }
+
+    Box(
+        modifier = modifier
+            .aspectRatio(1f)
+            .clip(CircleShape)
+            .background(backgroundColor)
+            .clickable(onClick = onClick),
+        contentAlignment = Alignment.Center
+    ) {
+        Text(
+            text = date.day.toString(),
+            textAlign = TextAlign.Center,
+            color = contentColor,
+            style = MaterialTheme.typography.bodyMedium
+        )
+    }
+}

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

@@ -0,0 +1,29 @@
+package plus.rua.project.ui
+
+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.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+
+@Composable
+fun MonthHeader(
+    year: Int,
+    month: Int,
+    weekNumber: Int,
+    modifier: Modifier = Modifier
+) {
+    Row(
+        modifier = modifier
+            .fillMaxWidth()
+            .padding(horizontal = 16.dp, vertical = 12.dp)
+    ) {
+        Text(
+            text = "${year}年${month}月 第${weekNumber}周",
+            style = MaterialTheme.typography.titleMedium
+        )
+    }
+}

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

@@ -0,0 +1,26 @@
+package plus.rua.project.ui
+
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.style.TextAlign
+
+private val WEEKDAY_LABELS = listOf("一", "二", "三", "四", "五", "六", "日")
+
+@Composable
+fun WeekdayHeader(modifier: Modifier = Modifier) {
+    Row(modifier = modifier.fillMaxWidth()) {
+        WEEKDAY_LABELS.forEach { label ->
+            Text(
+                text = label,
+                modifier = Modifier.weight(1f),
+                textAlign = TextAlign.Center,
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant
+            )
+        }
+    }
+}