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/auth/validation.dart

@@ -0,0 +1,35 @@
+import '../generated/i18n/app_localizations.dart';
+
+abstract final class AuthValidation {
+  static String? email(String? value, AppLocalizations l) {
+    final email = value?.trim() ?? '';
+    if (email.isEmpty) return l.emailRequired;
+    if (!RegExp(r'^[^\s@]+@[^\s@]+\.[^\s@]+$').hasMatch(email)) {
+      return l.emailInvalid;
+    }
+    return null;
+  }
+
+  static String? name(String? value, AppLocalizations l) =>
+      (value?.trim().isEmpty ?? true) ? l.nameRequired : null;
+
+  static String? password(
+    String? value,
+    AppLocalizations l, {
+    required bool signUp,
+  }) {
+    if (value == null || value.isEmpty) return l.passwordRequired;
+    if (signUp && value.length < 12) return l.passwordHint;
+    return null;
+  }
+
+  static String? confirmation(
+    String? value,
+    String password,
+    AppLocalizations l,
+  ) => value == null || value.isEmpty
+      ? l.confirmationRequired
+      : value != password
+      ? l.passwordMismatch
+      : null;
+}