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/page.tsx

@@ -3,6 +3,7 @@
 import { Building2, LayoutGrid, List, Map, MapPin } from 'lucide-react';
 import dynamic from 'next/dynamic';
 import Image from 'next/image';
+import { notFound } from 'next/navigation';
 import * as React from 'react';
 import { ProjectCard } from '@/components/du-an/project-card';
 import { ProjectFilterBar } from '@/components/du-an/project-filter-bar';
@@ -19,6 +20,7 @@ import {
   type SearchProjectsParams,
 } from '@/lib/du-an-api';
 import { useProjectsSearch } from '@/lib/hooks/use-du-an';
+import { useResidentialProjectsFlag } from '@/lib/hooks/use-residential-projects-flag';
 import { cn } from '@/lib/utils';
 
 const ProjectMap = dynamic(
@@ -31,6 +33,7 @@ const PAGE_SIZE = 12;
 type ViewMode = 'grid' | 'list' | 'map';
 
 export default function DuAnPage() {
+  const flagEnabled = useResidentialProjectsFlag();
   const [filters, setFilters] = React.useState<SearchProjectsParams>({
     page: 1,
     limit: PAGE_SIZE,
@@ -39,6 +42,10 @@ export default function DuAnPage() {
 
   const { data, isLoading, isError } = useProjectsSearch(filters);
 
+  if (!flagEnabled) {
+    notFound();
+  }
+
   const handleFilterChange = (newFilters: SearchProjectsParams) => {
     setFilters({ ...newFilters, limit: PAGE_SIZE });
     window.scrollTo({ top: 0, behavior: 'smooth' });