Show centered node join/leave overlay for 5s (v1.0.4)

- Replaces bottom toast: dark rounded card centered on screen,
  auto-hides after 5 seconds
- Optional test trigger via node_toast_test intent extra

app/src/main/res/layout/activity_settings.xml

@@ -139,6 +139,15 @@
             android:padding="12dp"
             android:hint="9, 11, 12, 13" />
 
+        <CheckBox
+            android:id="@+id/chk_node_toasts"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:layout_marginBottom="24dp"
+            android:text="@string/pref_node_toasts"
+            android:textColor="@color/text_primary"
+            android:textSize="15sp" />
+
         <Button
             android:id="@+id/btn_save"
             android:layout_width="match_parent"