implemented a comprehensive client-side demo mode triggered by a "demo@" username prefix, serving curated fixture data for all app modules

lib/api/demo/demo_mode.dart

@@ -0,0 +1,27 @@
+import '../../model/account_data.dart';
+
+/// Central switch for the client-side demo mode.
+///
+/// A login whose username starts with [usernamePrefix] (the password is
+/// ignored) enters demo mode: instead of talking to the real backends, the app
+/// serves curated fixtures from `DemoData` for every feature. This lets Google
+/// Play reviewers and automated screenshot runs see a fully populated app
+/// without a real account and without any network dependency.
+///
+/// The flag is persisted through [AccountData.isDemo], so a demo session
+/// survives cold starts exactly like a normal login. It works in release builds
+/// too — reviewers run the shipped release build.
+class DemoMode {
+  const DemoMode._();
+
+  /// Usernames starting with this prefix trigger demo mode. Whatever the
+  /// reviewer types as the password is accepted.
+  static const String usernamePrefix = 'demo@';
+
+  /// Whether [username], as typed on the login screen, should enter demo mode.
+  static bool matches(String username) =>
+      username.trim().toLowerCase().startsWith(usernamePrefix);
+
+  /// True while the active session is a demo session.
+  static bool get active => AccountData().isDemo;
+}