feat(auth): implement AuthGate using StreamBuilder for session tracking

Author: hoanghaoz

src/lib/features/auth/presentation/view/auth_gate.dart

@@ -0,0 +1,35 @@
+import 'package:flutter/material.dart';
+import 'package:supabase_flutter/supabase_flutter.dart';
+import 'login_view.dart';
+import '../../../main/view/screens/main_screen.dart';
+
+class AuthGate extends StatelessWidget {
+  const AuthGate({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    // StreamBuilder continously checks the auth state
+    return StreamBuilder<AuthState>(
+      stream: Supabase.instance.client.auth.onAuthStateChange,
+      builder: (context, snapshot) {
+        // Wait response from Supabase
+        if (snapshot.connectionState == ConnectionState.waiting) {
+          return const Scaffold(
+            body: Center(child: CircularProgressIndicator()),
+          );
+        }
+
+        // Check if there is an active session ( user logged in )
+        final session = snapshot.data?.session;
+
+        // if session exists -> Navigate to MainScreen
+        if (session != null) {
+          return const MainScreen();
+        }
+
+        // if session not exists -> Navigate to LoginView
+        return const LoginView();
+      },
+    );
+  }
+}