chore: remove admin SPA and update to two-app production setup

Remove apps/admin/ entirely — platform admin functionality now lives
in apps/app/ under /platform/* routes for super_admin users.

Production URL scheme changed:
- Organizer app: crewli.app (was app.crewli.app)
- Portal: portal.crewli.app (unchanged)
- API: api.crewli.app (unchanged)
- admin.crewli.app and app.crewli.app retired

Backend:
- Removed FRONTEND_ADMIN_URL config and admin cookie (crewli_admin_token)
  from SetAuthCookie, CookieBearerToken, cors.php, app.php
- Updated .env and .env.example (two origins, no port 5173)
- Updated cookie test: admin origin test → unknown origin fallback test

Infrastructure:
- Makefile: removed admin target
- deploy/nginx: updated CSP comment, removed admin vhost
- Updated README.md, CLAUDE.md, and all dev-docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/config/app.php

@@ -123,7 +123,6 @@ return [
         'store' => env('APP_MAINTENANCE_STORE', 'database'),
     ],
 
-    'frontend_admin_url' => env('FRONTEND_ADMIN_URL', 'http://localhost:5173'),
     'frontend_app_url' => env('FRONTEND_APP_URL', 'http://localhost:5174'),
     'frontend_portal_url' => env('FRONTEND_PORTAL_URL', 'http://localhost:5175'),