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>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help services services-stop api admin app portal
+.PHONY: help services services-stop api app portal
 
 # Colors
 GREEN  := \033[0;32m
@@ -18,7 +18,6 @@ help:
 	@echo ""
 	@echo "  $(YELLOW)Development Servers:$(NC)"
 	@echo "    make api              Laravel API        → http://localhost:8000"
-	@echo "    make admin            Admin Dashboard    → http://localhost:5173"
 	@echo "    make app              Organizer SPA      → http://localhost:5174"
 	@echo "    make portal           Portal SPA         → http://localhost:5175"
 	@echo ""
@@ -49,10 +48,6 @@ api:
 	@echo "$(GREEN)Starting Laravel API → http://localhost:8000$(NC)"
 	@cd api && php artisan serve
 
-admin:
-	@echo "$(GREEN)Starting Admin SPA → http://localhost:5173$(NC)"
-	@cd apps/admin && pnpm dev
-
 app:
 	@echo "$(GREEN)Starting Organizer SPA → http://localhost:5174$(NC)"
 	@cd apps/app && pnpm dev