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/widgets/locale.dart

@@ -0,0 +1,54 @@
+import 'package:flutter/material.dart';
+
+import 'package:provider/provider.dart';
+
+import '../providers/locale.dart';
+
+import '../generated/i18n/app_localizations.dart';
+
+class LocaleSelector extends StatelessWidget {
+  const LocaleSelector({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final provider = context.watch<LocaleProvider>();
+    return PopupMenuButton<Locale>(
+      key: const ValueKey('locale-selector'),
+      tooltip: AppLocalizations.of(context)!.language,
+      onSelected: (locale) async {
+        final saved = await provider.setLocale(locale);
+        if (!saved && context.mounted) {
+          ScaffoldMessenger.of(context).showSnackBar(
+            SnackBar(
+              content: Text(AppLocalizations.of(context)!.preferenceNotSaved),
+            ),
+          );
+        }
+      },
+      itemBuilder: (_) => AppLocalizations.supportedLocales
+          .map(
+            (locale) => CheckedPopupMenuItem(
+              value: locale,
+              checked: locale == provider.locale,
+              child: Text(locale.languageCode == 'ru' ? 'Русский' : 'English'),
+            ),
+          )
+          .toList(),
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 14),
+        child: Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            const Icon(Icons.language, size: 20),
+            const SizedBox(width: 8),
+            Text(
+              provider.locale.languageCode.toUpperCase(),
+              style: Theme.of(context).textTheme.labelLarge,
+            ),
+            const Icon(Icons.expand_more, size: 20),
+          ],
+        ),
+      ),
+    );
+  }
+}