Next.js route groups like (dashboard) don't appear in URLs, so the
sidebar, header, and dashboard page were linking to /dashboard/inbox,
/dashboard/contacts, etc. which don't exist and returned 404.
Fixed paths in:
- src/components/layout/sidebar.tsx (nav hrefs)
- src/components/layout/header.tsx (page title map)
- src/app/(dashboard)/dashboard/page.tsx (router.push calls)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>