docs: update company-vendor management documentation

- Document all admin UI pages (list, detail, edit)
- Document transfer ownership modal with user search
- Add user management API endpoints section
- Update ownership transfer description
- Fix encoding issue in marketplace-integration.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/frontend/overview.md

@@ -163,7 +163,7 @@ All three frontends share the same core technologies:
 | Backend | FastAPI | REST API + routing |
 | Templates | Jinja2 | Server-side rendering |
 | Interactivity | Alpine.js 3.x | Client-side reactivity |
-| Styling | Tailwind CSS 2.x | Utility-first CSS (CDN + fallback) |
+| Styling | Tailwind CSS 2.2.x | Utility-first CSS (CDN + fallback) - [Build Guide](tailwind-css.md) |
 | Icons | Heroicons | SVG icon system |
 | HTTP Client | Fetch API | API requests |
 | State Management | Alpine.js reactive | No external state lib |