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/app/CyreneRoot.kt

@@ -1,9 +1,16 @@
 package top.yeij.cyrene.app
 
+import android.app.Activity
+import android.content.Context
+import android.content.ContextWrapper
+import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.runtime.SideEffect
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
+import androidx.compose.ui.platform.LocalView
+import androidx.core.view.WindowCompat
 import top.yeij.cyrene.core.prefs.AppPrefs
 import top.yeij.cyrene.core.prefs.LocalAppPrefs
 import top.yeij.cyrene.core.prefs.ThemeMode
@@ -11,11 +18,28 @@ import top.yeij.cyrene.core.theme.CyreneTheme
 import top.yukonga.miuix.kmp.theme.ColorSchemeMode
 
 /**
- * 应用根：读 App 偏好 → 决定 Miuix 主题模式 → 通过 [LocalAppPrefs] 下发给渲染层。
+ * 应用根：读 App 偏好 → 决定 Miuix 主题模式 → 通过 [LocalAppPrefs] 下发给渲染层，
+ * 同时把系统栏图标明暗跟着主题走（否则浅色模式下白字白底看不见）。
  */
 @Composable
 fun CyreneRoot(container: AppContainer) {
     val prefs by container.appSettings.data.collectAsState(initial = AppPrefs())
+    val darkTheme = when (prefs.themeMode) {
+        ThemeMode.System -> isSystemInDarkTheme()
+        ThemeMode.Light -> false
+        ThemeMode.Dark -> true
+    }
+
+    val view = LocalView.current
+    if (!view.isInEditMode) {
+        SideEffect {
+            val window = view.context.findActivity()?.window ?: return@SideEffect
+            WindowCompat.getInsetsController(window, view).apply {
+                isAppearanceLightStatusBars = !darkTheme
+                isAppearanceLightNavigationBars = !darkTheme
+            }
+        }
+    }
 
     CompositionLocalProvider(LocalAppPrefs provides prefs) {
         CyreneTheme(mode = prefs.themeMode.toColorSchemeMode()) {
@@ -29,3 +53,9 @@ private fun ThemeMode.toColorSchemeMode(): ColorSchemeMode = when (this) {
     ThemeMode.Light -> ColorSchemeMode.Light
     ThemeMode.Dark -> ColorSchemeMode.Dark
 }
+
+private tailrec fun Context.findActivity(): Activity? = when (this) {
+    is Activity -> this
+    is ContextWrapper -> baseContext.findActivity()
+    else -> null
+}

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 {