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/providers/locale.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/material.dart';
+
+import 'package:shared_preferences/shared_preferences.dart';
+
+class LocaleProvider extends ChangeNotifier {
+  static const storageKey = 'sxchnge_last_locale';
+  LocaleProvider({Locale initialLocale = const Locale('en'), this.preferences})
+    : _locale = resolve(initialLocale.languageCode, const []);
+
+  Locale _locale;
+  final SharedPreferences? preferences;
+  Future<bool> _pendingSave = Future.value(true);
+  Locale get locale => _locale;
+
+  static Locale resolve(String? saved, List<Locale> systemLocales) {
+    final code =
+        saved?.split(RegExp('[-_]')).first ??
+        (systemLocales.isEmpty ? 'en' : systemLocales.first.languageCode);
+    return Locale(code == 'ru' ? 'ru' : 'en');
+  }
+
+  static Future<LocaleProvider> load(List<Locale> systemLocales) async {
+    try {
+      final prefs = await SharedPreferences.getInstance();
+      return LocaleProvider(
+        initialLocale: resolve(prefs.getString(storageKey), systemLocales),
+        preferences: prefs,
+      );
+    } catch (_) {
+      return LocaleProvider(initialLocale: resolve(null, systemLocales));
+    }
+  }
+
+  Future<bool> setLocale(Locale value) {
+    final next = resolve(value.languageCode, const []);
+    if (next == _locale) return _pendingSave;
+    _locale = next;
+    notifyListeners();
+    // Serialize writes so rapid language changes persist the last selection.
+    _pendingSave = _pendingSave.then((_) async {
+      try {
+        return await preferences?.setString(storageKey, next.languageCode) ??
+            false;
+      } catch (_) {
+        return false;
+      }
+    });
+    return _pendingSave;
+  }
+}