fix(android): polish app chrome and join guard

apps/chanora_flutter/android/app/src/main/kotlin/app/chanora/chanora_flutter/MainActivity.kt

@@ -1,7 +1,10 @@
 package app.chanora.chanora_flutter
 
 import android.content.Context
+import android.graphics.Color
+import android.os.Build
 import android.os.Bundle
+import android.view.View
 import app.chanora.chanora_flutter.AndroidPermissionRequester
 import app.chanora.chanora_flutter.BackIntentBridge
 import app.chanora.chanora_flutter.MethodChannels
@@ -76,6 +79,11 @@ class MainActivity : FlutterActivity() {
 
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
+        window.navigationBarColor = Color.rgb(255, 251, 254)
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+            window.decorView.systemUiVisibility =
+                window.decorView.systemUiVisibility or View.SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR
+        }
         // SDD-105: pass the Application context to the Rust side so the
         // audio engine can open device handles. Must run on the main thread
         // before any `chanora_audio` call from Dart. The native library is