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/main.dart

@@ -0,0 +1,71 @@
+import 'package:flutter/material.dart';
+
+import 'package:provider/provider.dart';
+
+import 'package:sxchnge_auth/sxchnge_auth.dart';
+
+import 'package:sxchnge_theme/sxchnge_theme.dart';
+
+import 'providers/locale.dart';
+import 'routing/router.dart';
+
+import 'generated/i18n/app_localizations.dart';
+
+Future<void> main() async {
+  final binding = WidgetsFlutterBinding.ensureInitialized();
+  final locale = await LocaleProvider.load(binding.platformDispatcher.locales);
+  runApp(SxchngeApp(localeProvider: locale));
+}
+
+class SxchngeApp extends StatefulWidget {
+  const SxchngeApp({
+    super.key,
+    this.localeProvider,
+    this.loginService = const DummyLoginService(),
+    this.signupService = const DummySignupService(),
+    this.verificationService = const DummyVerificationService(),
+  });
+  final LocaleProvider? localeProvider;
+  final LoginService loginService;
+  final SignupService signupService;
+  final VerificationService verificationService;
+
+  @override
+  State<SxchngeApp> createState() => _SxchngeAppState();
+}
+
+class _SxchngeAppState extends State<SxchngeApp> {
+  late final _locale = widget.localeProvider ?? LocaleProvider();
+
+  late final _router = createAppRouter();
+
+  @override
+  void dispose() {
+    _router.dispose();
+    _locale.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) => MultiProvider(
+    providers: [
+      ChangeNotifierProvider.value(value: _locale),
+      Provider<VerificationService>.value(value: widget.verificationService),
+      ChangeNotifierProvider(create: (_) => LoginProvider(widget.loginService)),
+      ChangeNotifierProvider(
+        create: (_) => SignupProvider(widget.signupService),
+      ),
+    ],
+    child: Consumer<LocaleProvider>(
+      builder: (context, locale, _) => MaterialApp.router(
+        onGenerateTitle: (context) => AppLocalizations.of(context)!.appTitle,
+        debugShowCheckedModeBanner: false,
+        theme: SxchngeTheme.light(),
+        locale: locale.locale,
+        localizationsDelegates: AppLocalizations.localizationsDelegates,
+        supportedLocales: AppLocalizations.supportedLocales,
+        routerConfig: _router,
+      ),
+    ),
+  );
+}