fix: enter immersive mode after setContentView to avoid DecorView NPE

app/src/main/java/top/yeij/hearth/MainActivity.kt

@@ -54,9 +54,13 @@ class MainActivity : Activity() {
 
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
-        Log.d(TAG, "onCreate: enter immersive mode")
-        enterImmersive()
+        Log.d(TAG, "onCreate: setup webview then enter immersive mode")
+        // 先 setContentView 创建 DecorView，再进沉浸式；否则 window.insetsController 会
+        // 因 DecorView 尚未创建（null）而 NPE 崩溃
+        // Set content view first to create the DecorView, then enter immersive mode;
+        // otherwise window.insetsController throws NPE because DecorView is not created yet
         setupWebView()
+        enterImmersive()
         checkNotificationAccess()
     }