docs: remove admin SPA references and update production URLs

The admin SPA (apps/admin/) has been retired. Its functionality now
lives in apps/app/ under /platform/* routes for super_admin users.
Updated all documentation to reflect: 2 SPAs instead of 3, removed
FRONTEND_ADMIN_URL/port 5173 references, changed production URL from
app.crewli.app to crewli.app. Retired admin-specific security audit
findings (A13-2, A13-4, A13-5, A13-7) and APPS-01 backlog item.

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

dev-docs/MASTER_PROMPT_CURSOR.md

@@ -15,9 +15,8 @@ Crewli — multi-tenant SaaS for event/festival management.
 
 - **Frontend:** Vue 3 + TypeScript + Vuexy 9.5 (Vuetify 3.10) + Pinia +
   TanStack Vue Query + VeeValidate + Zod
-- **Three SPAs:**
-  - `apps/admin/` — Super Admin (port 5173)
-  - `apps/app/` — Organizer main app (port 5174)
+- **Two SPAs:**
+  - `apps/app/` — Organizer + Platform Admin main app (port 5174)
   - `apps/portal/` — Dual-mode portal (port 5175)
 - **API base:** `VITE_API_URL` from `.env.local` (default `http://localhost:8000`)
 - **Axios instance:** `src/lib/axios.ts` — this is the ONLY axios instance.