fix(native): back from another tab goes home before closing the app

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/js/core/native-bridge.js

@@ -24,12 +24,14 @@ export function init() {
     onNavTap: handleNavTap,
     // The shell's QR scanner answers requestScan() here: { text } or { error }.
     onScanResult: handleScanResult,
-    // Android's back button: one scene back while there is one; false lets
-    // the shell close the app instead.
+    // Android's back button: one scene back while there is one, then home
+    // (the Play tab); false only there, and the shell closes the app.
     onBack: () => {
       const before = scene.currentSceneId();
       scene.pop();
-      return scene.currentSceneId() !== before;
+      if (scene.currentSceneId() !== before) return true;
+      if (scene.getCurrentWorld() !== 'play') { scene.switchWorld('play'); return true; }
+      return false;
     },
   };