Completely remove Firebase dependencies and convert to 100% pure local standalone mode

src/App.tsx

@@ -25,7 +25,7 @@ import {
   saveNotificationLog, 
   fetchCalendarEvents, 
   saveCalendarEvent 
-} from './lib/firebase';
+} from './lib/localStorageEngine';
 
 import { Navbar } from './components/Navbar';
 import { Sidebar } from './components/Sidebar';