feat: Android 13+ 预测性返回手势（Predictive Back）

- BackHandler 升级为 PredictiveBackHandler expect/actual
- Android 13+ 启用系统级预测返回，跟手阶段同步位移/缩放页面
- Android 低版本回退至普通 BackHandler
- iOS 保持空实现（无系统返回手势）
- 页面返回动画统一 250ms 时长，提升流畅感
- AndroidManifest 启用 enableOnBackInvokedCallback

androidApp/src/main/AndroidManifest.xml

@@ -3,6 +3,7 @@
 
     <application
         android:allowBackup="true"
+        android:enableOnBackInvokedCallback="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"

shared/src/androidMain/kotlin/plus/rua/project/Platform.android.kt

@@ -2,6 +2,9 @@ package plus.rua.project
 
 import android.os.Build
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.rememberCoroutineScope
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.launch
 
 class AndroidPlatform : Platform {
     override val name: String = "Android ${Build.VERSION.SDK_INT}"
@@ -14,6 +17,25 @@ actual fun getGifUri(gifFile: String): String = "file:///android_asset/gifs/$gif
 actual fun getAppIconUri(): String = "file:///android_asset/app_icon.png"
 
 @Composable
-actual fun BackHandler(enabled: Boolean, onBack: () -> Unit) {
-    androidx.activity.compose.BackHandler(enabled = enabled, onBack = onBack)
+actual fun PredictiveBackHandler(
+    enabled: Boolean,
+    onProgress: (Float) -> Unit,
+    onBack: () -> Unit,
+    onCancel: () -> Unit
+) {
+    if (Build.VERSION.SDK_INT >= 34) {
+        val scope = rememberCoroutineScope()
+        androidx.activity.compose.PredictiveBackHandler(enabled) { progress ->
+            try {
+                progress.collect { backEvent ->
+                    onProgress(backEvent.progress)
+                }
+                onBack()
+            } catch (e: CancellationException) {
+                onCancel()
+            }
+        }
+    } else {
+        androidx.activity.compose.BackHandler(enabled = enabled, onBack = onBack)
+    }
 }

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

@@ -1,6 +1,7 @@
 package plus.rua.project
 
 import androidx.compose.animation.AnimatedContent
+import androidx.compose.animation.core.tween
 import androidx.compose.animation.fadeIn
 import androidx.compose.animation.fadeOut
 import androidx.compose.animation.slideInHorizontally
@@ -13,10 +14,12 @@ import androidx.compose.material3.darkColorScheme
 import androidx.compose.material3.lightColorScheme
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.tooling.preview.Preview
 import plus.rua.project.ui.AboutScreen
 import plus.rua.project.ui.CalendarMonthView
@@ -31,6 +34,20 @@ private enum class Screen { Main, About, Licenses }
 @Preview(name = "Calendar App")
 fun App() {
     var currentScreen by remember { mutableStateOf(Screen.Main) }
+    var backProgress by remember { mutableFloatStateOf(0f) }
+
+    val handleBack: () -> Unit = {
+        backProgress = 0f
+        when (currentScreen) {
+            Screen.About -> currentScreen = Screen.Main
+            Screen.Licenses -> currentScreen = Screen.About
+            else -> {}
+        }
+    }
+
+    val handleCancel: () -> Unit = {
+        backProgress = 0f
+    }
 
     val colorScheme = if (isSystemInDarkTheme()) darkColorScheme() else lightColorScheme()
     MaterialTheme(colorScheme = colorScheme) {
@@ -38,13 +55,13 @@ fun App() {
             targetState = currentScreen,
             transitionSpec = {
                 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
+                    // 正向导航：新页面从右侧滑入覆盖，旧页面略微左移+淡出
                     (slideInHorizontally { it } + fadeIn()) togetherWith
                         (slideOutHorizontally { -it / 4 } + fadeOut())
                 } else {
-                    // 向后导航：新页面从左侧滑入，旧页面略微右移+淡出
-                    (slideInHorizontally { -it } + fadeIn()) togetherWith
-                        (slideOutHorizontally { it / 4 } + fadeOut())
+                    // 返回导航：新页面从左侧滑入，旧页面向右侧滑出
+                    (slideInHorizontally(animationSpec = tween(250)) { -it } + fadeIn(animationSpec = tween(250))) togetherWith
+                        (slideOutHorizontally(animationSpec = tween(250)) { it } + fadeOut(animationSpec = tween(250)))
                 }
             },
             modifier = Modifier.fillMaxSize()
@@ -55,16 +72,36 @@ fun App() {
                     onNavigateToAbout = { currentScreen = Screen.About }
                 )
                 Screen.About -> {
-                    BackHandler { currentScreen = Screen.Main }
+                    PredictiveBackHandler(
+                        enabled = backProgress == 0f,
+                        onProgress = { backProgress = it },
+                        onBack = handleBack,
+                        onCancel = handleCancel
+                    )
                     AboutScreen(
                         onBack = { currentScreen = Screen.Main },
-                        onNavigateToLicenses = { currentScreen = Screen.Licenses }
+                        onNavigateToLicenses = { currentScreen = Screen.Licenses },
+                        modifier = Modifier.graphicsLayer {
+                            translationX = backProgress * size.width * 0.3f
+                            scaleX = 1f - backProgress * 0.05f
+                            scaleY = 1f - backProgress * 0.05f
+                        }
                     )
                 }
                 Screen.Licenses -> {
-                    BackHandler { currentScreen = Screen.About }
+                    PredictiveBackHandler(
+                        enabled = backProgress == 0f,
+                        onProgress = { backProgress = it },
+                        onBack = handleBack,
+                        onCancel = handleCancel
+                    )
                     LicensesScreen(
-                        onBack = { currentScreen = Screen.About }
+                        onBack = { currentScreen = Screen.About },
+                        modifier = Modifier.graphicsLayer {
+                            translationX = backProgress * size.width * 0.3f
+                            scaleX = 1f - backProgress * 0.05f
+                            scaleY = 1f - backProgress * 0.05f
+                        }
                     )
                 }
             }

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

@@ -19,10 +19,17 @@ expect fun getGifUri(gifFile: String): String
 expect fun getAppIconUri(): String
 
 /**
- * 拦截系统返回手势。
+ * 预测性返回手势处理器（Android 13+）。
  *
- * @param enabled 是否启用拦截
- * @param onBack 返回回调
+ * @param enabled 是否启用
+ * @param onProgress 手势进度回调（0.0~1.0），跟手过程中持续调用
+ * @param onBack 手势完成回调（滑动距离足够，执行返回）
+ * @param onCancel 手势取消回调（滑动距离不足，回弹）
  */
 @Composable
-expect fun BackHandler(enabled: Boolean = true, onBack: () -> Unit)
+expect fun PredictiveBackHandler(
+    enabled: Boolean = true,
+    onProgress: (Float) -> Unit = {},
+    onBack: () -> Unit,
+    onCancel: () -> Unit = {}
+)

shared/src/iosMain/kotlin/plus/rua/project/Platform.ios.kt

@@ -15,6 +15,11 @@ actual fun getGifUri(gifFile: String): String = "compose.resource://files/$gifFi
 actual fun getAppIconUri(): String = "compose.resource://files/app_icon.png"
 
 @Composable
-actual fun BackHandler(enabled: Boolean, onBack: () -> Unit) {
-    // iOS 没有系统返回键，由导航栏按钮处理
+actual fun PredictiveBackHandler(
+    enabled: Boolean,
+    onProgress: (Float) -> Unit,
+    onBack: () -> Unit,
+    onCancel: () -> Unit
+) {
+    // iOS 没有预测性返回手势，由导航栏按钮处理
 }