feat(web): residential_projects feature flag for /du-an routes (TEC-2757)

- Add useResidentialProjectsFlag hook with NEXT_PUBLIC_FEATURE_RESIDENTIAL_PROJECTS env + URL/localStorage override (mirrors AVM v2 pattern)
- Gate /du-an index (client) and /du-an/[slug] detail (server) routes via notFound() when flag disabled
- Add component tests for index page including disabled-flag notFound branch

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/app/[locale]/(public)/du-an/[slug]/page.tsx

@@ -2,12 +2,15 @@ import type { Metadata } from 'next';
 import { notFound } from 'next/navigation';
 import { DuAnDetailClient } from '@/components/du-an/du-an-detail-client';
 import { fetchProjectBySlug } from '@/lib/du-an-server';
+import { isResidentialProjectsEnabledServer } from '@/lib/hooks/use-residential-projects-flag';
 
 interface PageProps {
   params: Promise<{ slug: string; locale: string }>;
 }
 
 export async function generateMetadata({ params }: PageProps): Promise<Metadata> {
+  if (!isResidentialProjectsEnabledServer()) return { title: 'Không tìm thấy dự án' };
+
   const { slug } = await params;
   const project = await fetchProjectBySlug(slug);
   if (!project) return { title: 'Không tìm thấy dự án' };
@@ -27,6 +30,10 @@ export async function generateMetadata({ params }: PageProps): Promise<Metadata>
 }
 
 export default async function DuAnDetailPage({ params }: PageProps) {
+  if (!isResidentialProjectsEnabledServer()) {
+    notFound();
+  }
+
   const { slug } = await params;
   const project = await fetchProjectBySlug(slug);