feat: 页面切换添加滑入滑出动效

使用 AnimatedContent 为 Main/About/Licenses 导航添加方向感知的
slide + fade 转场动画，向前导航从右滑入，返回从左滑入。

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

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

@@ -1,6 +1,13 @@
 package plus.rua.project
 
+import androidx.compose.animation.AnimatedContent
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.slideInHorizontally
+import androidx.compose.animation.slideOutHorizontally
+import androidx.compose.animation.togetherWith
 import androidx.compose.foundation.isSystemInDarkTheme
+import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.darkColorScheme
 import androidx.compose.material3.lightColorScheme
@@ -27,23 +34,39 @@ fun App() {
 
     val colorScheme = if (isSystemInDarkTheme()) darkColorScheme() else lightColorScheme()
     MaterialTheme(colorScheme = colorScheme) {
-        when (currentScreen) {
+        AnimatedContent(
-            Screen.Main -> CalendarMonthView(
+            targetState = currentScreen,
-                modifier = Modifier,
+            transitionSpec = {
-                onNavigateToAbout = { currentScreen = Screen.About }
+                if (targetState.ordinal > initialState.ordinal) {
-            )
+                    // 向前导航：新页面从右侧滑入覆盖，旧页面略微左移+淡出
-            Screen.About -> {
+                    (slideInHorizontally { it } + fadeIn()) togetherWith
-                BackHandler { currentScreen = Screen.Main }
+                        (slideOutHorizontally { -it / 4 } + fadeOut())
-                AboutScreen(
+                } else {
-                    onBack = { currentScreen = Screen.Main },
+                    // 向后导航：新页面从左侧滑入，旧页面略微右移+淡出
-                    onNavigateToLicenses = { currentScreen = Screen.Licenses }
+                    (slideInHorizontally { -it } + fadeIn()) togetherWith
-                )
+                        (slideOutHorizontally { it / 4 } + fadeOut())
-            }
+                }
-            Screen.Licenses -> {
+            },
-                BackHandler { currentScreen = Screen.About }
+            modifier = Modifier.fillMaxSize()
-                LicensesScreen(
+        ) { screen ->
-                    onBack = { currentScreen = Screen.About }
+            when (screen) {
+                Screen.Main -> CalendarMonthView(
+                    modifier = Modifier,
+                    onNavigateToAbout = { currentScreen = Screen.About }
                 )
+                Screen.About -> {
+                    BackHandler { currentScreen = Screen.Main }
+                    AboutScreen(
+                        onBack = { currentScreen = Screen.Main },
+                        onNavigateToLicenses = { currentScreen = Screen.Licenses }
+                    )
+                }
+                Screen.Licenses -> {
+                    BackHandler { currentScreen = Screen.About }
+                    LicensesScreen(
+                        onBack = { currentScreen = Screen.About }
+                    )
+                }
             }
         }
     }