Add CalendarMonthPage with 6x7 day grid layout

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

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

@@ -0,0 +1,71 @@
+package plus.rua.project.ui
+
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import kotlinx.datetime.DatePeriod
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.minus
+import kotlinx.datetime.plus
+
+@Composable
+fun CalendarMonthPage(
+    year: Int,
+    month: Int,
+    selectedDate: LocalDate,
+    today: LocalDate,
+    onDateClick: (LocalDate) -> Unit,
+    modifier: Modifier = Modifier
+) {
+    val days = remember(year, month) {
+        generateMonthDays(year, month)
+    }
+
+    Column(modifier = modifier) {
+        WeekdayHeader(modifier = Modifier.fillMaxWidth())
+
+        days.chunked(7).forEach { week ->
+            Row(
+                modifier = Modifier
+                    .fillMaxWidth()
+                    .padding(vertical = 2.dp)
+            ) {
+                week.forEach { dayData ->
+                    DayCell(
+                        date = dayData.date,
+                        isCurrentMonth = dayData.isCurrentMonth,
+                        isSelected = dayData.date == selectedDate,
+                        isToday = dayData.date == today,
+                        onClick = { onDateClick(dayData.date) },
+                        modifier = Modifier.weight(1f)
+                    )
+                }
+            }
+        }
+    }
+}
+
+private data class DayData(
+    val date: LocalDate,
+    val isCurrentMonth: Boolean
+)
+
+@Suppress("DEPRECATION")
+private fun generateMonthDays(year: Int, month: Int): List<DayData> {
+    val firstOfMonth = LocalDate(year, month, 1)
+    val offset = firstOfMonth.dayOfWeek.ordinal
+    val startDate = firstOfMonth.minus(DatePeriod(days = offset))
+
+    return (0 until 42).map { i ->
+        val date = startDate.plus(DatePeriod(days = i))
+        DayData(
+            date = date,
+            isCurrentMonth = date.monthNumber == month && date.year == year
+        )
+    }
+}