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/lib/hooks/use-residential-projects-flag.ts

@@ -0,0 +1,59 @@
+'use client';
+
+import { useEffect, useState } from 'react';
+
+const LOCAL_STORAGE_KEY = 'goodgo:residential_projects';
+const QUERY_PARAM = 'residential_projects';
+
+function readEnvDefault(): boolean {
+  const raw = process.env['NEXT_PUBLIC_FEATURE_RESIDENTIAL_PROJECTS'];
+  if (!raw) return false;
+  return raw === '1' || raw.toLowerCase() === 'true';
+}
+
+function readOverride(): boolean | null {
+  if (typeof window === 'undefined') return null;
+
+  const params = new URLSearchParams(window.location.search);
+  const qp = params.get(QUERY_PARAM);
+  if (qp === '1' || qp === 'true') {
+    try {
+      window.localStorage.setItem(LOCAL_STORAGE_KEY, '1');
+    } catch {
+      // localStorage may be blocked — ignore
+    }
+    return true;
+  }
+  if (qp === '0' || qp === 'false') {
+    try {
+      window.localStorage.setItem(LOCAL_STORAGE_KEY, '0');
+    } catch {
+      // localStorage may be blocked — ignore
+    }
+    return false;
+  }
+
+  try {
+    const stored = window.localStorage.getItem(LOCAL_STORAGE_KEY);
+    if (stored === '1') return true;
+    if (stored === '0') return false;
+  } catch {
+    // ignore
+  }
+  return null;
+}
+
+export function useResidentialProjectsFlag(): boolean {
+  const [enabled, setEnabled] = useState<boolean>(readEnvDefault());
+
+  useEffect(() => {
+    const override = readOverride();
+    setEnabled(override ?? readEnvDefault());
+  }, []);
+
+  return enabled;
+}
+
+export function isResidentialProjectsEnabledServer(): boolean {
+  return readEnvDefault();
+}