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/feedback.dart

@@ -0,0 +1,49 @@
+import 'package:flutter/material.dart';
+
+import 'package:sxchnge_auth/sxchnge_auth.dart';
+
+import '../generated/i18n/app_localizations.dart';
+
+class AuthFeedback extends StatelessWidget {
+  const AuthFeedback({super.key, required this.state, required this.signUp});
+  final AuthState state;
+  final bool signUp;
+
+  @override
+  Widget build(BuildContext context) {
+    final l = AppLocalizations.of(context)!;
+    final colors = Theme.of(context).colorScheme;
+    final error = state.error;
+    final message = error == null
+        ? (state.result!.isSimulated
+              ? (signUp ? l.signupPreview : l.loginPreview)
+              : l.authCompleted)
+        : switch (error) {
+            AuthError.invalidCredentials => l.authInvalidCredentials,
+            AuthError.emailInUse => l.authEmailInUse,
+            AuthError.unavailable => l.authUnavailable,
+            AuthError.unexpected => l.authUnexpected,
+          };
+    return Semantics(
+      liveRegion: true,
+      child: Container(
+        key: ValueKey(error == null ? 'preview-result' : 'auth-error'),
+        padding: const EdgeInsets.all(16),
+        decoration: BoxDecoration(
+          color: error == null
+              ? colors.secondaryContainer
+              : colors.errorContainer,
+          borderRadius: BorderRadius.circular(14),
+        ),
+        child: Text(
+          message,
+          style: Theme.of(context).textTheme.bodyMedium!.copyWith(
+            color: error == null
+                ? colors.onSecondaryContainer
+                : colors.onErrorContainer,
+          ),
+        ),
+      ),
+    );
+  }
+}