Reapply "M3: stub db sentinel in ws/memory mode so legacy guards pass"

This reverts commit d1ee69a70a.

src/App.js

@@ -122,10 +122,13 @@ const initializeStorage = () => {
   }
 
   // ws / memory mode: stub auth so App's anon-sign-in path works.
+  // db stays a truthy sentinel object so legacy `if (!db) return` guards pass;
+  // all real reads/writes route through `storage.*`, never the SDK `db`.
   const FAKE_USER = { uid: 'local-user', isAnonymous: true };
   auth = {
     currentUser: FAKE_USER,
   };
+  db = { __localStub: true };
   storage = getStorage();
   return true;
 };