Add CalendarViewModel with ISO week calculation and month grid generation

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

shared/src/commonMain/kotlin/plus/rua/project/CalendarViewModel.kt

@@ -0,0 +1,76 @@
+package plus.rua.project
+
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.setValue
+import kotlinx.datetime.DatePeriod
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.daysUntil
+import kotlinx.datetime.minus
+import kotlinx.datetime.plus
+import kotlinx.datetime.todayIn
+import kotlin.time.Clock
+
+data class CalendarDay(
+    val date: LocalDate,
+    val isCurrentMonth: Boolean,
+    val isToday: Boolean,
+    val isSelected: Boolean
+)
+
+class CalendarViewModel {
+    private val today: LocalDate = Clock.System.todayIn(TimeZone.currentSystemDefault())
+
+    var selectedDate by mutableStateOf(today)
+        private set
+
+    val currentYear: Int get() = selectedDate.year
+    @Suppress("DEPRECATION")
+    val currentMonth: Int get() = selectedDate.monthNumber
+
+    fun selectDate(date: LocalDate) {
+        selectedDate = date
+    }
+
+    fun getIsoWeekNumber(date: LocalDate): Int {
+        val jan4 = LocalDate(date.year, 1, 4)
+        val jan4DayOfWeek = jan4.dayOfWeek.ordinal
+        val week1Monday = jan4.minus(DatePeriod(days = jan4DayOfWeek))
+        val diff = week1Monday.daysUntil(date)
+        val weekNumber = diff / 7 + 1
+        return if (weekNumber < 1) {
+            getIsoWeekNumber(LocalDate(date.year - 1, 12, 28))
+        } else if (weekNumber > getIsoWeeksInYear(date.year)) {
+            1
+        } else {
+            weekNumber
+        }
+    }
+
+    private fun getIsoWeeksInYear(year: Int): Int {
+        val dec28 = LocalDate(year, 12, 28)
+        val jan4 = LocalDate(year, 1, 4)
+        val jan4DayOfWeek = jan4.dayOfWeek.ordinal
+        val week1Monday = jan4.minus(DatePeriod(days = jan4DayOfWeek))
+        val diff = week1Monday.daysUntil(dec28)
+        return diff / 7 + 1
+    }
+
+    @Suppress("DEPRECATION")
+    fun getMonthDays(year: Int, month: Int): List<CalendarDay> {
+        val firstOfMonth = LocalDate(year, month, 1)
+        val dayOfWeekOffset = firstOfMonth.dayOfWeek.ordinal
+        val startDate = firstOfMonth.minus(DatePeriod(days = dayOfWeekOffset))
+
+        return (0 until 42).map { i ->
+            val date = startDate.plus(DatePeriod(days = i))
+            CalendarDay(
+                date = date,
+                isCurrentMonth = date.monthNumber == month && date.year == year,
+                isToday = date == today,
+                isSelected = date == selectedDate
+            )
+        }
+    }
+}