feat: add auth screens and state management

lib/auth/phone_input_screen.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/material.dart';
+import 'package:intl_phone_field/intl_phone_field.dart';
+
+class PhoneInputScreen extends StatelessWidget {
+  const PhoneInputScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Enter Phone Number')),
+      body: Padding(
+        padding: const EdgeInsets.all(16.0),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: [
+            IntlPhoneField(
+              decoration: const InputDecoration(
+                labelText: 'Phone Number',
+                border: OutlineInputBorder(borderSide: BorderSide()),
+              ),
+              initialCountryCode: 'US',
+              onChanged: (phone) {
+                // TODO: Handle phone number changes
+              },
+            ),
+            const SizedBox(height: 20),
+            ElevatedButton(onPressed: () {}, child: const Text('Send OTP')),
+          ],
+        ),
+      ),
+    );
+  }
+}