fix(ui): 浅色模式系统栏图标改深色；返回键逐层回退

CyreneRoot 用 WindowInsetsControllerCompat 跟随主题切换系统栏明暗；ChatHomeScreen 为设置页与抽屉各加 BackHandler，避免直接回桌面。

app/src/main/kotlin/top/yeij/cyrene/ui/chat/ChatHomeScreen.kt

@@ -1,5 +1,6 @@
 package top.yeij.cyrene.ui.chat
 
+import androidx.activity.compose.BackHandler
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
@@ -15,7 +16,10 @@ import top.yeij.cyrene.ui.sessions.SessionDrawerContent
 import top.yeij.cyrene.ui.sessions.SessionsViewModel
 import top.yeij.cyrene.ui.settings.SettingsScreen
 
-/** 聊天页 + 侧边栏（会话切换）+ 设置页。 */
+/**
+ * 聊天页 + 侧边栏（会话切换）+ 设置页。
+ * 返回键逐层回退：设置 → 聊天；抽屉打开时先关抽屉；都关着才交给系统（回桌面）。
+ */
 @Composable
 fun ChatHomeScreen(
     container: AppContainer,
@@ -28,6 +32,9 @@ fun ChatHomeScreen(
     var drawerOpen by remember { mutableStateOf(false) }
     var showSettings by remember { mutableStateOf(false) }
 
+    BackHandler(enabled = showSettings) { showSettings = false }
+    BackHandler(enabled = drawerOpen) { drawerOpen = false }
+
     if (showSettings) {
         SettingsScreen(container = container, onBack = { showSettings = false })
     } else {