feat(web): enable residential projects feature flag by default for MVP

Flip NEXT_PUBLIC_FEATURE_RESIDENTIAL_PROJECTS default from false to
true so /du-an and /du-an/[slug] render without requiring an env var
or ?residential_projects=1 query override. Kill-switch preserved —
set the env var to "0"/"false" to disable.

The homepage now advertises Dự án as a core feature; having the page
404 by default contradicted that positioning.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -7,8 +7,8 @@ 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';
+  if (raw == null || raw === '') return true;
+  return !(raw === '0' || raw.toLowerCase() === 'false');
 }
 
 function readOverride(): boolean | null {