feat: 新增 BackHandler expect/actual 拦截系统返回手势

Android 通过 androidx.activity.compose.BackHandler 接管返回键，
在关于页与许可页返回上一级；iOS 无系统返回键，actual 为空实现。

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

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

@@ -1,6 +1,7 @@
 package plus.rua.project
 
 import android.os.Build
+import androidx.compose.runtime.Composable
 
 class AndroidPlatform : Platform {
     override val name: String = "Android ${Build.VERSION.SDK_INT}"
@@ -10,4 +11,9 @@ actual fun getPlatform(): Platform = AndroidPlatform()
 
 actual fun getGifUri(gifFile: String): String = "file:///android_asset/gifs/$gifFile"
 
-actual fun getAppIconUri(): String = "file:///android_asset/app_icon.png"
+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)
+}

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

@@ -32,13 +32,19 @@ fun App() {
                 modifier = Modifier,
                 onNavigateToAbout = { currentScreen = Screen.About }
             )
-            Screen.About -> AboutScreen(
-                onBack = { currentScreen = Screen.Main },
-                onNavigateToLicenses = { currentScreen = Screen.Licenses }
-            )
-            Screen.Licenses -> LicensesScreen(
-                onBack = { 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 }
+                )
+            }
         }
     }
 }

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

@@ -1,5 +1,7 @@
 package plus.rua.project
 
+import androidx.compose.runtime.Composable
+
 interface Platform {
     val name: String
 }
@@ -14,4 +16,13 @@ expect fun getPlatform(): Platform
  */
 expect fun getGifUri(gifFile: String): String
 
-expect fun getAppIconUri(): String
+expect fun getAppIconUri(): String
+
+/**
+ * 拦截系统返回手势。
+ *
+ * @param enabled 是否启用拦截
+ * @param onBack 返回回调
+ */
+@Composable
+expect fun BackHandler(enabled: Boolean = true, onBack: () -> Unit)

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

@@ -1,5 +1,6 @@
 package plus.rua.project
 
+import androidx.compose.runtime.Composable
 import platform.UIKit.UIDevice
 
 class IOSPlatform : Platform {
@@ -11,4 +12,9 @@ actual fun getPlatform(): Platform = IOSPlatform()
 
 actual fun getGifUri(gifFile: String): String = "compose.resource://files/$gifFile"
 
-actual fun getAppIconUri(): String = "compose.resource://files/app_icon.png"
+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
+}