feat: introduce (authenticated) route group layout for auth-protected pages

docs/route-groups-auth.md

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+# Authenticated Route Groups Architecture
+
+## Overview
+Previously, pages requiring user authentication (such as `/account`, `/change-password`, `/verify-email`) manually rendered `<ReduxGateWrapper>` and `<ProtectedPageWrapper>` within their page components or local layouts.
+
+## New Route Group Pattern
+Next.js route groups `(authenticated)` allow sharing a root authenticated layout without altering the URL path structure:
+- `src/app/[locale]/(authenticated)/layout.tsx`: Gates all subroutes with `ReduxGateWrapper` and `ProtectedPageWrapper`.
+- Individual pages under `(authenticated)` automatically inherit authorization and Redux rehydration guards without redundant boilerplate.
+- Simplifies testing and guarantees uniform authentication checks across all protected subroutes.

src/app/[locale]/(authenticated)/layout.tsx

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+import * as React from 'react';
+import { ReduxGateWrapper } from '../../components/ReduxGateWrapper';
+import { ProtectedPageWrapper } from '../../components/ProtectedPageWrapper';
+
+/**
+ * Route group layout providing auth guarding and Redux hydration
+ * for all authenticated pages (e.g. account, change-password).
+ */
+export default function AuthenticatedLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}): React.ReactElement {
+  return (
+    <ReduxGateWrapper>
+      <ProtectedPageWrapper>{children}</ProtectedPageWrapper>
+    </ReduxGateWrapper>
+  );
+}