Deliver localized auth app with verification and go_router

Integrate themed login/signup, EN/RU localization, shared dummy providers, verification recovery screens and named URL routes. Store frontend guidelines, workflow skills, validation and delivery documentation. Related to #2, #3, #5 and #6.

frontend/app/lib/routing/router.dart

@@ -0,0 +1,48 @@
+import 'dart:convert';
+
+import 'package:go_router/go_router.dart';
+
+import '../auth/page.dart';
+import '../verification/page.dart';
+import 'routes.dart';
+
+GoRouter createAppRouter() => GoRouter(
+  // Signup email is an in-memory navigation extra, never browser-history data.
+  extraCodec: const _TransientExtras(),
+  routes: [
+    GoRoute(
+      path: '/',
+      name: AppRoutes.login,
+      builder: (context, state) => const AuthPage(),
+    ),
+    GoRoute(
+      path: '/verification-sent',
+      name: AppRoutes.verificationSent,
+      builder: (context, state) => VerificationPage(
+        email: state.extra is String ? state.extra as String : null,
+      ),
+    ),
+    GoRoute(
+      path: '/verify-email',
+      name: AppRoutes.verifyEmail,
+      builder: (context, state) =>
+          VerificationPage(token: state.uri.queryParameters['token'] ?? ''),
+    ),
+  ],
+  // Preserve the existing fallback, and keep the visible URL canonical.
+  onException: (context, state, router) => router.goNamed(AppRoutes.login),
+);
+
+class _TransientExtras extends Codec<Object?, Object?> {
+  const _TransientExtras();
+  @override
+  Converter<Object?, Object?> get encoder => const _DiscardExtra();
+  @override
+  Converter<Object?, Object?> get decoder => const _DiscardExtra();
+}
+
+class _DiscardExtra extends Converter<Object?, Object?> {
+  const _DiscardExtra();
+  @override
+  Object? convert(Object? input) => null;
+}