move xfay app secret to .env

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/test_env.dart

@@ -0,0 +1,18 @@
+import 'dart:io';
+
+/// Loads KEY=VALUE pairs from a `.env` file at the project root.
+/// Lines starting with `#` and blank lines are ignored.
+/// Returns an empty map if the file does not exist.
+Map<String, String> loadEnv() {
+  final file = File('.env');
+  if (!file.existsSync()) return {};
+  final result = <String, String>{};
+  for (final line in file.readAsLinesSync()) {
+    final trimmed = line.trim();
+    if (trimmed.isEmpty || trimmed.startsWith('#')) continue;
+    final idx = trimmed.indexOf('=');
+    if (idx < 1) continue;
+    result[trimmed.substring(0, idx).trim()] = trimmed.substring(idx + 1).trim();
+  }
+  return result;
+}