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/drawable/bg_node_toast.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+    <solid android:color="#E61B222A" />
+    <corners android:radius="24dp" />
+    <stroke android:width="1dp" android:color="#40FFFFFF" />
+</shape>