Add circular reveal animation to DayCell with animated state transitions

Replace hard-switched background/border modifiers with updateTransition-based
animated properties: revealProgress for circular indicator radius, contentColor
and selectedColor via animateColor, borderAlpha for today stroke. Use drawBehind
for custom circle drawing with tween(250, FastOutSlowInEasing) spec.

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

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

@@ -1,8 +1,10 @@
 package plus.rua.project.ui
 
-import androidx.compose.foundation.BorderStroke
-import androidx.compose.foundation.background
-import androidx.compose.foundation.border
+import androidx.compose.animation.animateColor
+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.clickable
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.aspectRatio
@@ -10,13 +12,22 @@ import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
-import androidx.compose.ui.unit.dp
+import androidx.compose.ui.draw.drawBehind
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.drawscope.Stroke
 import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.dp
 import kotlinx.datetime.LocalDate
 
+enum class DayCellState {
+    NORMAL, OTHER_MONTH, TODAY, SELECTED, SELECTED_TODAY
+}
+
 /**
  * 单个日期单元格，显示日期数字并支持选中/今天/非当月状态。
  *
@@ -36,26 +47,84 @@ fun DayCell(
     onClick: () -> Unit,
     modifier: Modifier = Modifier
 ) {
-    val contentColor = when {
-        isSelected && isToday -> MaterialTheme.colorScheme.onPrimaryContainer
-        isSelected -> MaterialTheme.colorScheme.onPrimary
-        isToday -> MaterialTheme.colorScheme.primary
-        !isCurrentMonth -> MaterialTheme.colorScheme.onSurface.copy(alpha = 0.38f)
-        else -> MaterialTheme.colorScheme.onSurface
+    val currentState = when {
+        isSelected && isToday -> DayCellState.SELECTED_TODAY
+        isSelected -> DayCellState.SELECTED
+        isToday -> DayCellState.TODAY
+        !isCurrentMonth -> DayCellState.OTHER_MONTH
+        else -> DayCellState.NORMAL
     }
 
+    val transition = updateTransition(targetState = currentState, label = "dayCell")
+
+    val revealProgress by transition.animateFloat(
+        transitionSpec = { tween(250, easing = FastOutSlowInEasing) },
+        label = "revealProgress"
+    ) { state ->
+        when (state) {
+            DayCellState.SELECTED, DayCellState.SELECTED_TODAY -> 1f
+            else -> 0f
+        }
+    }
+
+    val contentColor by transition.animateColor(
+        transitionSpec = { tween(250, easing = FastOutSlowInEasing) },
+        label = "contentColor"
+    ) { state ->
+        when (state) {
+            DayCellState.SELECTED_TODAY -> MaterialTheme.colorScheme.onPrimaryContainer
+            DayCellState.SELECTED -> MaterialTheme.colorScheme.onPrimary
+            DayCellState.TODAY -> MaterialTheme.colorScheme.primary
+            DayCellState.OTHER_MONTH -> MaterialTheme.colorScheme.onSurface.copy(alpha = 0.38f)
+            DayCellState.NORMAL -> MaterialTheme.colorScheme.onSurface
+        }
+    }
+
+    val selectedColor by transition.animateColor(
+        transitionSpec = { tween(250, easing = FastOutSlowInEasing) },
+        label = "selectedColor"
+    ) { state ->
+        when (state) {
+            DayCellState.SELECTED_TODAY -> MaterialTheme.colorScheme.primaryContainer
+            DayCellState.SELECTED -> MaterialTheme.colorScheme.primary
+            else -> Color.Transparent
+        }
+    }
+
+    val borderAlpha by transition.animateFloat(
+        transitionSpec = { tween(250, easing = FastOutSlowInEasing) },
+        label = "borderAlpha"
+    ) { state ->
+        when (state) {
+            DayCellState.TODAY -> 1.5f
+            else -> 0f
+        }
+    }
+
+    val todayBorderColor = MaterialTheme.colorScheme.primary
+
     Box(
         modifier = modifier
             .aspectRatio(1f)
             .clip(CircleShape)
-            .then(
-                when {
-                    isSelected && isToday -> Modifier.background(MaterialTheme.colorScheme.primaryContainer)
-                    isSelected -> Modifier.background(MaterialTheme.colorScheme.primary)
-                    isToday -> Modifier.border(BorderStroke(1.5.dp, MaterialTheme.colorScheme.primary), CircleShape)
-                    else -> Modifier
+            .drawBehind {
+                if (revealProgress > 0f) {
+                    val maxRadius = size.minDimension / 2f
+                    drawCircle(
+                        color = selectedColor,
+                        radius = revealProgress * maxRadius,
+                        center = Offset(size.width / 2f, size.height / 2f)
+                    )
                 }
-            )
+                if (borderAlpha > 0f) {
+                    drawCircle(
+                        color = todayBorderColor.copy(alpha = borderAlpha.coerceAtMost(1f)),
+                        radius = size.minDimension / 2f,
+                        center = Offset(size.width / 2f, size.height / 2f),
+                        style = Stroke(width = borderAlpha.coerceAtMost(1.5f) * 1.5.dp.toPx())
+                    )
+                }
+            }
             .clickable(onClick = onClick),
         contentAlignment = Alignment.Center
     ) {