feat: Implement initial authentication, multi-environment support, and core app infrastructure.

lib/core/storage/secure_storage.dart

@@ -0,0 +1,33 @@
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+class SecureStorage {
+  static const _storage = FlutterSecureStorage();
+
+  static const _accessTokenKey = 'access_token';
+  static const _refreshTokenKey = 'refresh_token';
+
+  static Future<void> saveTokens({
+    required String accessToken,
+    required String refreshToken,
+  }) async {
+    await Future.wait([
+      _storage.write(key: _accessTokenKey, value: accessToken),
+      _storage.write(key: _refreshTokenKey, value: refreshToken),
+    ]);
+  }
+
+  static Future<String?> getAccessToken() async {
+    return _storage.read(key: _accessTokenKey);
+  }
+
+  static Future<String?> getRefreshToken() async {
+    return _storage.read(key: _refreshTokenKey);
+  }
+
+  static Future<void> clearTokens() async {
+    await Future.wait([
+      _storage.delete(key: _accessTokenKey),
+      _storage.delete(key: _refreshTokenKey),
+    ]);
+  }
+}